A folded MachZehnder interferometer in a holetype silicon photonic crystal is proposed, which consists of one splitter and several mirrors. Light propagates between them employing selfcollimation effect. Its performance is investigated based on finitedifference timedomain simulation technique. The two complementary transmission spectra at two output ports are both in the shape of sinusoidal curves in the frequency range from 0.255c/a to 0.270c/a. The peak spacing becomes smaller when the length difference between the two interfering branches is increased. This folded MachZehnder interferometer with compact structure is much smaller than nonfolded interferencetype filters in photonic crystals. It may work as a wavelength division demultiplexer in highdensity photonic integrated circuits.