Effects of heat treatment on InP crystals have been studied. It was found that the carrier concentration of undoped InP can be decreased after annealing. The reduction of the carrier concentration depends on the carrier concentration of as-grown wafers. When lightly Zn doped (Zn concentration is 4.0 × 1015 cm-3) InP wafers were annealed, high resistivity (more than 105 Ω cm) InP could be obtained. The activation energy measured by temperature dependent Hall effect was 0.44 eV. The reason why the carrier concentration decreases seems to be concerned with the increase of the density of the 0.44 eV deep level defects. By applying these effects, low Fe doped semi-insulating InP wafers could also be obtained by annealing slightly Zn co-doped wafers.