We propose and demonstrate a compact optical-microwave phase detector taking advantage of polarization modulation. The proposed polarization modulator-based phase detector (PolM-PD) shows 460-attosecond residual timing jitter integrated from 1 Hz to 100 kHz when synchronizing an 8-GHz dielectric resonator oscillator to a free-running 250-MHz mode-locked Er-fiber laser. The absolute single-sideband phase noise of the locked 8-GHz DRO is -138 dBc/Hz (-165 dBc/Hz) at an offset frequency of 10 kHz (10 MHz). The proposed PolM-PD has a compact structure implemented by all commercially available components. Because no fiber loop and delay line are used, the proposed PolM-PD can be potentially integrated on a chip.