System in Package (SiP), also known as Multi-Chip Module (MCM), is a package with a number of integrated circuits enclosed in a single package or module. A SiP package performs all or most of the functions of an electronic system, and is typically used inside a mobile phone, digital music player, etc. Since a SiP package is large in size, non-uniform temperature distribution in a reflow machine during reflow process could occur and affect the reliability of a SiP package. Thus, accurate prediction and understanding the thermal behavior of a SiP package during the reflow process is necessary and important. The purpose of this paper was using thermal modeling to investigate the thermal response of the SiP product during the reflow process and validating with experiment measurements. A forced-convection reflow oven was modeled using computational fluid dynamic software (ANSYS Fluent), and structural heating on the SiP product was conducted using conjugate heat transfer of Fluent without any other software. The simulation has successfully showed the complex flow pattern in a reflow oven including characteristics of a free-jet region, a stagnation-flow region, a wall-jet region, a recirculation region and vortices. The simulation results were in good agreement with the experiment data. The methodology developed in this paper could not only be used for accurate prediction of the temperature distribution in an oven but also be used for designing a thermal profile for reflow soldering production with minimum amount of temperature variation.