The main objective of this work is to study numerically the unsteady natural convection phenomena in water filled rectangular enclosure with hot partitioned wall. The fluid flow and the heat transfer described in terms of continuity, linear momentum and energy equations were predicted by using the finite volume method. Streamlines, isotherms and local Nusselt number time evolution are presented for all investigated values. The aspect ratio of the geometry, Prandtl number are fixed at 0.24, 6.64, respectively, for different partitions lengths; however the Rayleigh number values were ranging from 106 to 3,77x109 in order to observe the transition regime. Representative results illustrating the effects of the partition length for the heat transfer and the thermal boundary layer are also reported and discussed. The obtained results show that the presence of the partition on the hot wall affects both heat transfer and fluid flow. It was found that the average Nusselt number increases with increase in the Rayleigh number. Also, as the dimensionless partition length increases, the flow speed within the partitioned enclosure decreases. Moreover, the features of the unsteady flow induced by the presence of partitions are characterized and the mechanisms responsible for the unsteadiness are discussed.