The effect of grain size on optical properties and designed Ag/p-NiOx/n-Si/Al-based Schottky diode have been investigated. The XRD patterns are used to calculate the grain size of NiOx film, which increases with increasing thickness. Fractal dimensions were estimated from the power spectral density (PSD) algorithm using AFM images. The band gaps were calculated from recorded transmission spectra, showing the reduction of band gap from 3.88 eV to 3.67 eV with increasing grain size. The effect of grain size on the performance of Ag/p-NiOx/n-Si/Al-based Schottky-diodes are analyzed through I-V measurement. The diode parameters of the designed device were calculated using the thermionic emission, Norde, and Cheung models. It is found that the reduction of barrier height of the Schottky diode from 0.95 eV to 0.84 eV with increasing grain size of NiOx films. It is found that the grain size of film strongly impacted on the diode parameters, such as ideality factor (η), potential barrier (φh), series resistance (Rs) performance.