Judging by the questions we see here regarding how to do serial communitions, I think Pawan's comments are way too simplistic.

Wong, go to the tutorials here and read up on serial port operation (8051, chapter 8). If you haven't already done so, first read the 8051 bible (see links) so you will understand the basics of the 8051 hardware.

From your question I'm guessing that you are trying to do in-system programming (ISP) of an 8051 that supports ISP through its UART. If this isn't the case, you need to restate your question more clearly. It would also be helpful if you'd state just which 8051 variant you are using.

Dennis