In the present work, the size dependent electronic and optical nonlinearities of InP/ZnS core/shell quantum dot are investigated. The impurity states in InP/ZnS core/shell quantum dot are studied taking into account the geometrical confinement effect. The energy eigen values and the exciton binding energies are found using variational formulism with two variational parameters. Some nonlinear optical properties are concentrated. The exciton binding energies, optical band gap and the optical gain, in InP/ZnS quantum dot, are obtained. The optical gain as a function of photon energy is found using compact density approach. The optical gain in any reduced dimensional semiconductor system proposes an essential information about the heterostructure. The larger exciton binding energy of InP/ZnS quantum dot implies the wider size tuning within the strong confinement region. It has been observed that InP/ZnS core /shell quantum dots are safer, low toxicity and environmental friendly and is considered to be an alternative to CdSe/ZnS based quantum dots which cannot be used for bio-medical applications.