As design rule of semiconductor decreases continuously, overlay error control gets more and more important and challenging. It is also true that On Product Overlay (OPO) of leading edge memory device shows unprecedented level of accuracy, owing to the development of precision optics, mechanic stage and alignment system with active compensation method. However, the heating of reticle and lens acts as a dominant detriment against further improvement of overlay. Reticle heating is more critical than lens heating in current advanced scanners because lens heating can be mostly compensated by feed-forward control algorithm. In recent years, the tools and technical ideas for reticle heating control are proposed and thought to reduce the reticle heating effect. Nevertheless, it is not still simple to predict the accurate heating amount and overlay. And it is required to investigate the parameters affecting reticle heating quantitatively. In this paper, the reticle pattern density and exposure dose are considered as the main contributors, and the effects are investigated through experiments. Mask set of various transmittance are prepared by changing pattern density. After exposure with various doses, overlay are measured and analyzed by comparing with reference marks exposed in heating free condition. As a result, it is discovered that even in the case of low dose and high transmittance, reticle heating is hardly avoidable. It is also shown that there is a simple relationship among reticle heating, transmittance and exposure dose. Based on this relationship, the reticle heating is thought to be predicted if the transmittance and dose are fixed.