Electronic and optical properties of InP in zincblende crystal structure are studied using the full potential linearized augmented plane wave plus local orbitals (FP-LAPW+lo) program. The complex dielectric function and optical constants, such as optical absorption coefficient, reflectivity, refractive index and extinction coefficient, are calculated, which are in good agreement with the experimental results, better than those from the full potential linear muffin-tin orbital (FP-LMTO) method. We also have explained the origin of the spectral peaks on the basis of the electronic band structures.