RF low power measurement using network analyzer

Does anyone know how to measure RF power (output power, 10 mW) of a power amplifier with a network analyzer? Someone suggested me that I can do this but I don't know how. Could someone give me some ideas? Thanks a lot!

Some Network Analyzers are capable to measure power but it's not so accurate..
Power Meter is much better

Agilent new veersion NA can measurement power more accurate than old version.
You should calibrate the NA before using. The calibration have two methods: one is using external signal generator, and the other is using the internal signal generator of NA.
After calibration, you can measure your signal power precisely.

First, be ABSOLUTELY sure you don't overdrive the receiver on the Network Analyzer. Check the NA's specs to make sure you don't blow out the front-end! (very SSSSSSSSS to fix)

The NA will give you forward gain very accurately. If you set the input source power to 0 dBm, then the gain value you read will be the output power in dBm. (input power + gain = output power... 0 dBm + 9 dB gain = 9 dBm). If you set your source power to -10 dBm, then the -10+gain will be your output power in dBm (-10 dBm + 9 dB gain = -1 dBm).