Abstract: The noise temperature measurements for the receiving modules, the sub-arrays and the whole active phased array are discussed in this paper. Based on the extension of IRE effective noise temperature definition and using the well-known " Y -factor " method, the effective noise temperature of the system or individual channels under certain prescribed conditions may be obtained. The effective input noise temperature (or gain) of the receiving modules may be different from each other in the discussion. The loss of the power combining networks, the unequal power combining, and the two level power combining networks are considered also.