Combined with a DV-Xa calculation, the formation and stability of SixN±y cluster ions produced by laser ablation technique were investigated.It was observed in the mass spectrum that larger cluster ions were composed of one of the basic units,SiN3or SiN,,and other stable cluster ions.The mass spectrum shows that the maxima correspond to these cluster ions whose masses are times of the atomic mass of Si. The odd-even variation related to the number of N atom is believed to be caused by the differences of intensity of the intensity of the basic units.