Hello Kuan
Check the following step by step
1:you must have 11.592 mhz xtal for your project
because with 12mhz xtal occur incorrect internal timing

2:If you have printed circuit board, check
gnd and vcc of max232 is connected to 8051's gnd and vcc

3:with 1200 instead 9600 baud rate maybe your problem be solved

4:be sure +/- of caps are correct connected.

5:if any solUtion is not be usefull get this code



MOV SBUF,A
LCALL SERIAL_TRANS


SERIAL_TRANS:
MOV TMOD,#20H
MOV TH1,#-12
MOV SCON,#50H
SETB TR1
HERE1:
JNB TI,HERE1
CLR TR1
CLR TI
RET