Dependence of the phosphorescence emission of Platinum phthalocyanine single crystals around 971 nm on the magnetic field and on the incident laser intensity are investigated. The energy splitting of the spin sublevels of the lowest excited triplet state is about 8.5 cm -1 in zero field and about 12.4 cm -1 in the external magnetic field of 50 kG. The T–T (triplet-triplet) annihilation rate is estimated to be about 1×10 -2 cm 3 /sec and the magnetic field dependence of the T–T annihilation reveals that the | 0> spin component of the triplet exciton plays the most important role. The temperature dependence of the phosphorescence at the low temperature range is also studied.