Nowadays, fan-out package is regarded as one of the latest and most potential technologies because it possesses lower cost, thinner profile, and better electrical performance and thermal performance. However, thermally induced warpage in the molding process is a critical issue due to the larger wafer or panel size, the shrinkage of epoxy mold compound (EMC) during the curing stage, and the mismatch of coefficient of thermal expansion (CTE) among the constituent materials during the cooling stage, which needs to be controlled effectively for successful subsequent process of the fan-out package. In this paper, a novel $320 x 320$-mm? panel-level fan-out package based on ``Die Last'' process is developed. A coreless substrate with redistribution layer is fabricated and bonded onto a low-CTE and high-glass-transition-temperature (Tg) FR4 carrier through thermal release film. The thermally induced warpage issue in the molding process is investigated. A warpage simulation method is presented and verified by Shadow Moir? experiment. The error between the simulation and experimental results is about 4.8%. For the warpage optimization analysis, the effect of geometry structure on the warpage is first investigated by the design of simulation approach. Full factor experiment is conducted, and Minitab statistical software is utilized to analyze the effect of the geometry structure on warpage. It is found that decreasing die thickness and molding thicker EMC can effectively decrease the warpage. Then, the effects of molding temperature and in-plane CTE of FR4 on warpage are studied, respectively. When molding temperature is 120 ?C and in-plane CTE of FR4 decreases to 10.5 ppm/?C, the thermally induced warpage in the molding process is only about 0.31 mm, thus subsequent process of fan-out package can be conducted successfully.@en