The nonlinear evolution of collisional Rayleigh-Taylor (CRT) instability in the F region of ionosphere is studied for two ions-electron (O+, NO+, and e-) plasma. The enhancements in NO+ are found inside the O+ depletions. During linear phase of instability, NO+ concentration is mainly controlled by chemical process while during nonlinear phase, it is controlled by CRT induced transport process. In spite the dominant chemical loss during initial phase, significant NO+ ions could survive owing to their shielding by O+ depletions. The seeding of long wave perturbation, the O+ scale height below the base and prereversal enhancement play important role in supplying significant NO+ ions onto the topside which could possibly match the observations.