By use of some atom parameters in optical-field-ionized (OFI) plasmas obtained from the extended Cowan program based on Maxwell to suit any electron distribution with the help of additional program, the peak gain coefficient of Ni-like Kr system is calculated for the ion densities of 1×10 17 cm -3 and 3.2×10 17 cm -3 respectively after considering Doppler broadening and natural broadening. The theoretic peak gain coefficients of 14 cm -1 and 141 cm -1 are obtained respectively.