This paper describes cooling performance of natural convection in thin enclosures such as LCD Monitor. Natural convection cooling is widely used in electronic equipment which cannot use fans in order to decrease power consumption and to avoid a generation of noise. However, sometimes a tilt angle of an enclosure of thin type electronic equipment are changed according to the operating environment by a user. Natural convection is generated by the relationship between the gravity and the generation of the buoyancy force by generation of the distribution of air density by temperature rise. Therefore by changing the tilt angle, the natural convection cooling performance is changed and an accurate thermal design may not be achieved. Therefore, an additional database about natural convection cooling performance in the tilted enclosure should be prepared. From these backgrounds, in this research, the cooling performance of natural convection of heat transfer enhancement device for electrical chips mounted in thin enclosure were investigated while changing the tilt angle of the enclosure. The investigations were performed by the 3D-CFD analysis with Boussinesq assumption. The reliability of the analysis was evaluated by the comparison with the experiment. Through the research, we clarified the relationship between the cooling performance, the design parameters of the enclosure and the tilt angle.