I have installed apache2 so I can visit a test page test.html by typing http://localhost/test.html or http://128.xxx.xxx.xxx/test.html without any problem.
However, if I try to access the test page from the other computer, then it says no page is found. I can ping that 128.xxx.xxx.xxx ip address from other computer, but I just cann't access http://128.xxx.xxx.xxx/test.html.

Any body has idea?

you have to modify your firewall to fix this problem because your firewall may drop all packet from another computer to your webserver

I am using Ubuntu, I don't know how to disable firewall.

run
#iptables -F
on console... thats it

it's better to run followed commands :
service iptables stop
to disable firewall

I tried it, and still not work!

can you tell more about the errors ? and what happened when you typed the command
service iptables stop
try a gain!!!!!!
and if it is still unsuccessful you add the command
chmod 755 /...../index.html

sorry you add the command chmod 755 /..../test.html instead of chmod 755 /.../index.html

I guess I know what you mean. You're afraid that the other computer not have permission to read the test.html file?
Let me try it again.
By the way, what's 755 means?
How can I change the permission to the whole folder?

for the
#service iptables stop
it says: command not found!

for the
#iptables -F

it just executes, and nothing is displayed. and didn't return any error message or nothing.

please try to get the command /sbin/iptables stop
the number 755 is means user can read change and execute the file
you can use it with folder

permission has now changed to allow read/write/execute
but still not work.

and for the iptables stop, I execute it and says the following:

kei@oca:~$ /sbin/iptables stop
Bad argument `stop'
Try `iptables -h' or 'iptables --help' for more information.
kei@oca:~$ iptables -h
iptables v1.3.1

Usage: iptables -[AD] chain rule-specification [options]
iptables -[RI] chain rulenum rule-specification [options]
iptables -D chain rulenum [options]
iptables -[LFZ] [chain] [options]
iptables -[NX] chain
iptables -E old-chain-name new-chain-name
iptables -P chain target [options]
iptables -h (print this help information)

Commands:
Either long or short options are allowed.
--append -A chain Append to chain
--delete -D chain Delete matching rule from chain
--delete -D chain rulenum
Delete rule rulenum (1 = first) from chain
--insert -I chain [rulenum]
Insert in chain as rulenum (default 1=first)
--replace -R chain rulenum
Replace rule rulenum (1 = first) in chain
--list -L [chain] List the rules in a chain or all chains
--flush -F [chain] Delete all rules in chain or all chains
--zero -Z [chain] Zero counters in chain or all chains
--new -N chain Create a new user-defined chain
--delete-chain
-X [chain] Delete a user-defined chain
--policy -P chain target
Change policy on chain to target
--rename-chain
-E old-chain new-chain
Change chain name, (moving any references)
Options:
--proto -p [!] proto protocol: by number or name, eg. `tcp'
--source -s [!] address[/mask]
source specification
--destination -d [!] address[/mask]
destination specification
--in-interface -i [!] input name[+]
network interface name ([+] for wildcard)
--jump -j target
target for rule (may load target extension)
--goto -g chain
jump to chain with no return
--match -m match
extended match (may load extension)
--numeric -n numeric output of addresses and ports
--out-interface -o [!] output name[+]
network interface name ([+] for wildcard)
--table -t table table to manipulate (default: `filter')
--verbose -v verbose mode
--line-numbers print line numbers when listing
--exact -x expand numbers (display exact values)
[!] --fragment -f match second or further fragments only
--modprobe=<command> try to insert modules using this command
--set-counters PKTS BYTES set the counter during insert/append
[!] --version -V print package version.
kei@oca:~$