This paper presents the thermal analysis and validation of high power LED system with simulation and experimental methods. A 3W high power LED package is mounted on MCPCB and heat sink, which is tested inside a (300 x 300 x 300mm) still air chamber with three different forward currents. Experimental measurement with Thermal Transient Tester (T3Ster) is performed to capture the thermal transient characteristic of the LED system. The simulation is conducted with FloEFD 12.1 CFD software, using two-resistor model the LED package is defined with thermal resistance at different forward current. From the validation the simulation result closely match the experimental result, with highest percentage error at 4.75%. The surrounding air temperature of the system is also studied in the simulation. Following the verification, LED system with and without rectangular fixture are simulated and compared. The model without fixture shows better thermal results. The flow trajectory plot shows the difference in air convection with and without fixture. The simulation shows fixture disrupts the natural convection in the chamber.