Boilers are one of most important parts of power plants which changes subcooled water to superheat vapor. Combustion chambers are one of main parts of plants which water pipes and burners make up its walls. In this paper, combustion chamber of boiler in Shahid Montazeri Power Plant is numerically analyzed. profile of heat flux delivered to walls, thermal stress due to this heat flux and effects of pipe wall thickness parameter increment on these stresses are calculated. In order to obtain the desired results a 3 dimensional model by using computational fluid dynamics for 200MW boiler of Shahid Montazeri Power Plant is simulated. All boiler combustion chamber compartments such as swirling burner with real dimensions of the plant are modeled. The geometry of plant combustion chamber is a prism and consists of twelve burner of swirling type which are placed in two rows on rear walls. Burners are simulated in their original dimensions. Burners are made of central and peripheral air inlet and fuel inlet in the center of central air inlet. Main aim of this paper is to calculate delivered heat flux to the walls of boiler combustion chamber and its applied thermal stresses. Hence, simulating combustion of natural gas with Eddy-Dissipation method in combustion chamber, delivered heat flux to the walls is obtained. In order to simulate turbulent flow in the chamber and radiation k-? and DO methods are used, respectively. Results of boiler combustion chamber simulation are compared to the real data of the plant and obtained errors are in acceptable range. Obtained results show that flow and thermal field in the combustion chamber can be simulated with good accuracy. Investigating delivered heat flux to the walls due to combustion chamber simulation, maximum of delivered heat flux is calculated. Maximum calculated heat flux for walls is in front of burners. Applying maximum delivered heat flux profile to the pipes, a single pipe and neighboring pipes, thermal stresses are calculated in Abaqus. Obtained results are compared with profile of applied heat flux to the pipes which shows that applied heat flux profile is in accordance with stress distribution along the tube. Results obtained by numerical simulations by increasing tube thickness are compared in three directions, r, ?, z. Results indicate that thermal stresses are decreased by increasing tube thickness. The reason for this can be validated by calculating the stress analytically. Keywords: Boiler, Eddy-Dissipation Method, Combustion, Thermal Stress, Heat Flux, DO Method, k-? Method