In this study, analysis of transient natural convection in a porous medium with a vertical fracture across it in a rectangular enclosure was performed experimentally and numerically. A number of thermocouples in distinctive distances in the model were used to measure temperature distribution and confirm the numerical simulations. A detailed CFD simulation was carried out, which was based on the finite volume method to extend our experimental observations. The onset of transient natural convection in the fractured porous media was observed by experimental setup and successfully simulated by numerical analysis. A modified Rayleigh number for the constant surface temperature at the bottom boundary condition of the model was calculated in this study (i.e., use of instead of K), which was near to the theoretical value of linear stability analysis. Moreover, the critical time of the onset convection in the fractured model was predicted. The average Nusselt number was estimated to be about 3.5 for this fractured porous model.