A thermoelectric power detector is proposed to integrate with MMICs and other planar connecting circuit structures at X-band (8-12 GHz). The thermoelectric power detectors without and with the tapered line are designed to use 50 Ω coplanar waveguides as commonly applied in MMIC devices. The sensitivity without the tapered line is about 0.224, 0.211, and 0.202 mV/mW at 8, 10, and 12 GHz, respectively. For the power detector with the tapered line, the sensitivity is about 0.326, 0.311, and 0.295 mV/mW, and it increases averagely about 46% compared with that without the tapered line. This reported work is focused on developing microwave power detectors to be integrated with MMICs and other planar connecting circuit structures.