Laser ablation of indium phosphorus was investigated in tandem TOF mass spectrometer. The odd-even rule related to the P atoms was observed, the mass spectra of cluster ions exhibits a maximum when the number of P atom is odd. Theoretical calculation for InxP+y(y=1-3) with DV-Xα method indicated that the clustcr formation rule was in accordance with the conjugation energy curve. The structure of special cluster ions with "magic number" was also disscused.