A novel method to measure small phase in micro-holography is discussed. It originated from Zernike phase contrast imaging technique, which has been a successful linear phase-to-intensity mapping approach toward phase object. Instead of phase-changing plate, an interference phase contrast technique that is based on in-line holography is utilized to obtain a hologram in the proposed method. The intensity distribution of hologram presents a monotone function with respect to phase distribution of phase object. Thus, the phase can be directly detected through hologram without phase unwrapping. In the work, the contrast of hologram is analyzed toward various phase difference between object wave and reference wave. An optimal solution to measure small phase of phase object is developed, which extends the measuring range of phase. Simulation on a computer is performed to validate theoretically the proposed method.