In the present study, phenanthrene degradation by yeast Candida tropicalis NN4 was optimized in the presence of biogenically synthesized ZnO nanoparticle using plant extract of Ocimum tenuiflorum and produced biosurfactant in the medium. The biosynthesized ZnO nanoparticles were characterized by UV-visible spectroscopy, Fourier transform infrared spectroscopy (FTIR) and Transmission Electron Microscopy (TEM) analysis. Biosurfactant producing ability of the yeast isolate was investigated by drop collapse test, emulsification index, methylene blue agar plate method, oil displacement test and lipase activity. The type of biosurfactant produced was sophorolipid which was characterized by FT-IR analysis. Response surface methodology (RSM), three-level three variables Box Behnken Design (BBD) were employed to optimize the factors viz., pH (7), NaCl concentration (1.5 gL-1) and ZnO nanoparticle concentration (0.02gL-1) for maximum phenanthrene degradation using C. tropicalis NN4. The TEM micrograph illustrated that the biosynthesized ZnO nanoparticle exhibited different nanostructures viz. tetragonal, spherical, hexagonal and triangular shaped with the average core diameter of 20 nm. The actual value (86.5 ± 0.02{\%}) was in close agreement with predicted value (86.5 ± 0.01{\%}) obtained by the RSM model indicating the validity of the RSM model. Phenanthrene degradation was confirmed through FTIR and GC-MS analysis. These results support the effectiveness of using RSM for maximum phenanthrene degradation.