In this study, a W-band out-of-phase active power divider, with continuously tunable power division ratio, has been presented. Out-of-phase signal is generated by the compact transform balun with accurate phase and amplitude response. To stabilise the phase difference in different power division ratio, phase-invariant cascode variable gain amplifier (VGA) is designed in the two paths of the active power divider and 4-bits proportional to absolute temperature current source is designed to control voltage of VGA. The active power divider is implemented in SiGe BiCMOS process with measured tunable power division ratio from 1:16 up to 16:1, the phase shift from 74 to 90 GHz is about 10° as the gain changes up to 12 dB, which guarantees a good phase-invariant performance.