It is difficult to detect and measure slow moving natural objects from a space-borne SAR observation. This paper presents a simple and efficient method to retrieve moving velocity from quad-pol SAR data acquired by dual receive antenna (DRA). The core idea is to exploit the slight observation time difference between different polarizations (H- and V-pol transmitted signals). The difference of scattering centers between different polarizations can be removed by azimuth phase differentiation. It was applied to TerraSAR-X quad-pol SAR over slow drifting sea ice in the Arctic Ocean. The SAR measured velocity was about 1.4 km/hr (or 0.37 m/s) to 2.4 km/hr (or 0.67 m/s). While co-pol data (HH- and VV-pol) are useful to detect slow drifting sea ice because of the strength of returned signals, cross-pol (HV- and VH-pol) data seems more effective to measure the velocity.