The paper carried out a simulation of unsteady number in order to investigate the unsteady natural convection around two heated horizontal cylinders in a round sealed water tank with isothermal walls by using numerical simulation, and studied the flow and heat transfer at different Rayleigh numbers(104 to 107) and different position. It also analyzed flow and temperature field and discussed the variation of average Nusselt number (Nuavg) of heated cylinders in different situations. The simulation result showed that flow changes so much with the growth of Rayleigh number which determines whether the dominant heat transfer is conduction or convection. It is also found that there is always an optimum distance between two cylinders which make heat transfer to reach the best under the situation of natural convection.