A numerical study is performed to investigate the transient heat transfer and flow characteristics of aluminum oxide (Al2O3) nanoparticles dispersed in 50:50 ethylene glycol/water (EG/W) base fluid in a multipass crossflow minichannel heat exchanger. The time dependent thermal responses of the system in a laminar regime are predicted by solving the conservation equations using the finite volume method and SIMPLE algorithm. The transient regime is caused by a step change of nanofluid mass flow rate at the inlet of the minichannel heat exchanger. This step change can be analogous with a thermostat operation. In this study, three volume fractions up to 3 percent of Al2O3 nanoparticles dispersed to the base fluid EG/W are modeled and analyzed. In the numerical simulation, Al2O3-EG/W nanofluid is considered as a homogenous single-phase fluid. An analysis of the transient response for the variation of nanofluids volume concentrations is conducted. The study and corresponding analysis of results and discussions provide essential insights of the minichannel heat exchanger which might be useful to improve the design and cooling performance of thermal control systems. It is observed from the results that the nanofluid possesses a greater convective heat transfer compared to that of the base fluid. The enhancement of heat transfer increases with the increase of particle volume concentration. It is also found that the nanofluid with higher volume fraction approaches quasi-steady condition earlier than that of base fluid.