I have successfully upgrade but I can't boot from Fedora7. However, I can boot from fedora core 6 which is selected from boot menu list.

I paste the related information:
==========================================
# /sbin/blkid
/dev/mapper/VolGroup00-LogVol01: TYPE="swap"
/dev/mapper/VolGroup00-LogVol00: UUID="828418eb-aa98-4848-887d-b7bbcea8ebde" SEC_TYPE="ext2" TYPE="ext3"
/dev/hdb1: TYPE="ntfs"
/dev/hdb5: TYPE="ntfs"
/dev/hdb6: LABEL="D DISK" UUID="2C5C-1D93" TYPE="vfat"
/dev/hdd1: LABEL="/boot" UUID="c500cfbc-4a8d-4520-be26-1f4c180e4188" SEC_TYPE="ext2" TYPE="ext3"
/dev/hdd2: TYPE="swap"
/dev/VolGroup00/LogVol01: TYPE="swap"
/dev/VolGroup00/LogVol00: UUID="828418eb-aa98-4848-887d-b7bbcea8ebde" SEC_TYPE="ext2" TYPE="ext3"
############################################

# cat /etc/fstab
/dev/VolGroup00/LogVol00 / ext3 defaults 1 1
LABEL=/boot /boot ext3 defaults 1 2
devpts /dev/pts devpts gid=5,mode=620 0 0
tmpfs /dev/shm tmpfs defaults 0 0
proc /proc proc defaults 0 0
sysfs /sys sysfs defaults 0 0
/dev/VolGroup00/LogVol01 swap swap defaults 0 0
===============================================
# cat /boot/grub/grub.conf
# grub.conf generated by anaconda
#
# Note that you do not have to rerun grub after making changes to this file
# NOTICE: You have a /boot partition. This means that
# all kernel and initrd paths are relative to /boot/, eg.
# root (hd1,0)
# kernel /vmlinuz-version ro root=/dev/VolGroup00/LogVol00
# initrd /initrd-version.img
#boot=/dev/hdb
default=0
timeout=5
splashimage=(hd1,0)/grub/splash.xpm.gz
hiddenmenu
title Fedora (2.6.21-1.3194.fc7)
root (hd1,0)
kernel /vmlinuz-2.6.21-1.3194.fc7 ro root=/dev/VolGroup00/LogVol00 rhgb quiet
initrd /initrd-2.6.21-1.3194.fc7.img
title Fedora Core (2.6.20-1.2952.fc6)
root (hd1,0)
kernel /vmlinuz-2.6.20-1.2952.fc6 ro root=/dev/VolGroup00/LogVol00 rhgb quiet
initrd /initrd-2.6.20-1.2952.fc6.img
title Windows XP
rootnoverify (hd0,0)
chainloader +1
===========================================

I can boot from fc6 but not fedora 7. of course I can boot windows XP.
Looking forward to hearing from you
Thanks in advance!

Fedora Core 6 is booting from (hd1,0). If you look at your grub menu Fedora 7 is trying to boot from the same partition. That's not going to work. If Fedora Core 6 works and boots successfully then Fedora 7 must need to boot from another partition. Change the line for Fedora 7 that reads:
root (hd1,0)
to read
root (hd2,0)
and see if it boots.

Actually, the two kernels co-exist in the same drive, which happened before when I upgraded the kernel. I select either new kernel or older kernel, on which Linux works well. I think the Fedora 7 may need the "LABEL" to boot system rather than the device ID refer to: Upgrade Related Issues. . But I am not sure how to do that.

Seeking more help!

Not too familiar with LVM. Never used it. Just a guess, how about ro root=/dev/VolGroup00/LogVol01 rhgb quiet instead of ro root=/dev/VolGroup00/LogVol00 rhgb quiet ?? Seems I've read/seen a lot of posts regarding LVM and problems booting. I stick with EXT3. Most of the more experienced forum members seem to think that LVM is more of a curse than a blessing.

I had exactly the same problem, I upgraded also from Fedora Core 6 to Fedora 7. After I edited the /etc/grub.conf by removing

title Fedora Core (2.6.20-1.2952.fc6)
root (hd1,0)
kernel /vmlinuz-2.6.20-1.2952.fc6 ro root=/dev/VolGroup00/LogVol00 rhgb quiet
initrd /initrd-2.6.20-1.2952.fc6.img

The system seems not to have any problems, the only problem I have now is that Fedora 7 will hang when I try to logout, shutdown or reboot

I deleted the FC6 item and it then works although it is still errors during boot.