Gamma ray spectra were measured by a Φ8"×4"NaI(Tl) detector with different radioisotope sources to test a full-energy peak fitting function

which is a Gaussian plus a high-energy tail correction and a low-energy cor-rection.The fitting function can properly describe a γ-ray peak with nine parameters

which are automatically opti-mized.The parameters for 57Co

137Cs

60Co and 40K and the corresponding analysis were given.Considering requests on analysis time and credibility of the γ spectra by a NaI whole body counter

the function was simplified on the basis of experiment accuracy.The high-energy tail correction term can be ignored due to lower contribution

the peak ad-dress and FWHM of the Gaussian

and the low-energy tail correction term

could be bonded together to reduce the freedom to four parameters in the fitting.The results show that the Analytic Improved Figure-of-Merit(AIFOM) of the fitting for γ-ray peaks of 57Co

137Cs

60Co and 40K measured by the NaI detector are less than 0.01%.