about the VCO's measure.

I am designing a LC-VCO,would you give me any advice on the following questions:
(1)How to calculate the output impedance of LV-VCO? Because I should do the impedance matching when measurement.
(2)I ever saw that the buffer and Bias-T are added into the LC-VCO for measure on IEEE papers.What the role of the buffer and Bias-T?

Any calculation of circuit parameters requires first of all knowing the circuit. Your questions are abstract ones and with posts like yours, nobody can expect concrete answers but only teorethical.

You can measeure a VCO without connecting it directly to a spectrum analyzer. Use a probe made out of a broken coax cable and sniff out the radiation. This way, you have nothing loading down and pulling your VCO. When a buffer and bias tees are used, they should be very high impedance at the terminals interfacing the VCO. This way, the buffer and bias-T are invisible to the VCO and won't pull its frequency.