I have a PC with 2 NIC. each NIC connected to another PC just like this A<--->R<--->B.
(R) contains 2 sniffers each of them connected to different NIC. each sniffer(1) replace destination MAC address located in each packet coming from A with B's MAC address.
sniffer (2) does the opposite process which make R work just like a router.
(A) and (B) run TCP chat program.
Next for each incoming packet from (A) i did the following:
I appended 1 byte of data to the TCP data section.

• I incremented Total length located in Ip header of packet by one
• I recomputed check sum located in IP header

but the packet doesn't reach B, and A continue retransmitting that packet, later it reset the connection.
can anybody show me what the problem?

I notice your post history shows at least three duplicate threads and one warning for the same.
I'm giving you a second and final warning not to post duplicate threads again.