Natural convection in two-dimensional square enclosure containing adiabatic cylinder at the center has been studied numerically using finite element method. In the present study, top wall is considered adiabatic, two vertical walls are maintained at constant cold temperature, a constant heat flux is embedded at the bottom wall and the non-heated parts of the bottom wall are considered adiabatic. The aim of this work is to demonstrate the capabilities of this numerical methodology for handling such problems. The finite element formulations of the dimensionless governing equations with the associated boundary conditions are solved by a nonlinear coupled solution algorithm using three-nodded triangular element discretization scheme for all the field variables. The Grashof number is varied from 103 to 106 and Prandtl number is taken as 0.71. The effect of heat source length on the fluid flow and heat transfer process in the enclosure are analyzed. Results are presented in the form of streamline and isotherm plots.