An enclosure with a moving lid is commonly used in heat and mass transmission. Also, many investigations have been done so far on a mixed convective flow in a cavity, but no research has been done to observe the low Reynolds number effect in the presence of MHD and radiation in a cavity with a semi-circular heater incorporated kerosine oil-based CNT nanofluid. In addition, oil-based nanofluid makes the working fluid more stable at a higher temperature. This study numerically investigates the time-dependent effect of low Reynolds number on Kerosene oil-based CNT nanofluid with magnetic field and radiation. The governing equations were employed with the finite element method based Galerkin residual technique. Brownian motion of nanoparticles was considered to determine the thermal conductivity and dynamic viscosity. Lower values of Reynolds number are taken, such as 50 to 200 with fixed values of radiation parameter, the particle concentration, the Hartmann number, and the Richardson number. The results were illustrated as heat transfer and fluid flow for three dimensionless time conditions. Results indicate that increasing the fluid velocity improves the Nusselt number and drag force. The vorticity rises to 54% while increasing the fluid velocity, however, the pressure gradient and average temperature become lower. It is also found that the average fluid velocity is 2.2 times higher in the Re = 50 than in the Re = 100. For the time dependency of this study, the thermo-hydrodynamics behavior changes with dimensionless time. Finally, this study would be a guide for designing thermal devices related to heat transfer, especially using the Kerosene oil-based CNT nanofluid under different conditions.