Abstract: Load-Pull t of microwave power transistor is a very meaningful and foundational job. A Si bipolar junction transistor （BJT） with operating frequency of 1.2 - 1.4GHz is measured using the Load-Pull automatic measurement system. At first the basic Load-Pull measurement principles on large-signal parameters of power transistors and measurement system setup are introduced briefly. Then the main technique used in measurement is described detailedly, and large-signal parameters of the transistor are extracted. The measured results show that the valuable power contour of a transistor can be obtained by Load-Pull measurement, which can provide a reference for the design of high power and wide-band matching network.