Crystallization of ammonium polyvanadate is a key procedure for the production of vanadium pentoxide. The kinetics of nucleation of ammonium polyvanadate and the effects of impurities on the nucleation including sodium ion, potassium ion, hexavalent chromium ion, dihydrogen phosphate were studied in this research by using conductivity method. The solubility of ammonium polyvanadate was determined first, then the nucleation parameters were obtained by combining induction period with the classical primary nucleation theory. The results show the nucleation is homogeneous when with a high supersaturation degree of higher than 25. The critical nucleation radius, free energy and activation energy decrease, and the nucleation rate increases, with the increase of supersaturation degree. The existence of sodium ion, potassium ion, dihydrogen phosphate in the solution will prolong the induction period of crystallizing ammonium polyvanadate, with phosphorus showing the most significant influence. The morphology of the ammonium polyvanadate crystals will change from flake to spherical- like with the existence of phosphorus.