Due to the superior transport properties of nanofluids, they are highly recommended in Hydrodynamic Journal bearing for improving load-carrying capacity and system miniaturization. But, it also increases the power loss and accumulation of particles due to high viscosity and particle-drag force. The present investigation is to study the impact of nanofluids on improving the performance of plain hydrodynamic journal bearing. The three water-based Nano-particles (WS2, SiO2, and Al2O3) are considered at various speeds and concentrations. The analysis is divided into two parts: The first part deals with the modeling of single-phase considering nanofluids as homogeneous fluid with effective properties. The numerical results reveal that Alumina Nano-particle with 2% volume fraction increases load capacity per unit power loss by 21% and reduces side leakage by 13% at 500 rpm speed. While at high speed and Nano-particle concentration, the slope of the pressure distribution curve reduces drastically, nanofluid's efficacy reduces, and penalties increase. The second part concerns modeling a Multi-Phase Mixture model to track variation in nanofluid concentration due to Brownian and thermophoresis motion of nanofluid, owning to viscous heating at a high shear rate. The ultimate objective of the current work is to optimize Nano-particle volume fraction to enhance the performance of the Hydrodynamic Journal bearing.