Gamma radiation-induced birefringence (RIB) variations in a solid core photonic crystal polarization-maintaining fiber (Pc-PMF) and a Panda PMF (Pa-PMF) are investigated. Experiments show that the birefringence of the fibers changes approximately linearly with a radiation dose over the range from 0 to 400 krad(Si), with negative slope coefficients of 8.91×10-10/krad(Si) and 4.38×10-9/krad(Si) for the Pc-PMF and Pa-PMF, respectively. Models of RIB variations for the two fibers are established, and the theoretical results agree well with the experimental ones.