This study is concerned with studying the performance of SiO2–water nanofluid flow through a three-dimensional straight mini-channel with different values of aspect ratio (AR) of (0.5, 1.0, and 1.6) and a fixed hydraulic diameter under a uniform heat flux. The governing equations are developed and solved numerically using the finite volume method for a single-phase flow with standard Kappa-Epsilon (ҡ–ɛ) turbulence model via a user-defined function (UDF) over the Reynolds number (Re) range of (10,000–35,000). Numerical results indicated that the average Nusselt number ratio increases as the Reynolds number and volume concentration of the nanoparticles increase for all values of the channel aspect ratio. The results indicated that the maximum enhancement of the heat transfer coefficient (benefit) achieved is 94.69% at AR = 0.5, along with the lowest increase of pressure drop (penalty) of 13.1%. The highest performance evaluation criterion (PEC) of 1.64 is found at AR = 0.5, Re = 35,000, and 5% concentration.