The excitation functions of the evaporation residue formation probability of three heavy nuclei ¹⁹⁴Pb, ²⁰⁰Pb and ²⁰⁶Pb are calculated by using a Langevin equation coupled with a statistical decay model. The results show that the neutron-to-proton ratio (N/Z) of a compound nucleus has an effect on survival probability and this effect becomes larger with increasing N/Z. This is because the fission barrier and the pre-saddle particle emission depend on the N/Z ratio of the system.