Liquid slip at hydrophobic surfaces in microchannels has frequently been observed. We present here an analytical solution for oscillating flow in a parallel-plate microchannel by combining electrokinetic transport phenomena with Navier's slip condition. Results suggest that the apparent and true zeta potential for a typical electrolye solution can differ by more than an order of magnitude with only a 1% slip. From our analysis, we propose a new method to measure simultaneously the true zeta potential and slip coefficient.