We propose a compact, high efficient full photonic crystal Mach-Zehnder (MZ) interferometer based on the self-collimation and photonic band gap in two-dimensional photonic crystals. Line defected photonic crystals are used as the beam splitter and the mirror. The interference theory is used to discuss the interferometer output mechanism, and compared with the finite-difference time-domain (FDTD) simulation results. The designed MZ interferometer can surve as micro-detectors of gas and liquid, which may play an important role in integrated optics.