Living small animals and phantoms were irradiated with neutrons in a radiation field formed by a 238Pu-Be source. Then a Nal（Tl） detector installed in a lead shielding room was used to measure the characteristic γ-rays originated by nuclear reaction 31P（n

α）28A1. The net integral counts of full- energy peaks from animals and phantoms were compared and the phosphorus content was calculated. An almost linear relation between the net integral counts and phosphorus content in phantoms was observed. The phosphorus percentage were measured for twenty mice by both IVNAA and ultra-violet spectroscopic analysis. Their differences were within 18%.