Although several computer simulation codes have been developed to predict and prevent defects of lost foam castings, most of them are focused on aluminum alloy castings. This paper presents a new simulation method suitable for not only aluminum alloy but also cast iron. First of all, main assumptions and numerical schemes are as below: 1) Gas flow in the mold follows the D’Arcy’s law. 2) The gas pressure in the kinetic zone (gas gap) is uniform in each gas group and is solved by using the flow field of melt one time-step before. 3) The gas obeys the ideal gas law. 4) Latent heats of evaporative pattern degradation are simplified by an overall latent heat in which both melting and evaporation are considered. 5) The pattern absorbs heat of melt through conduction, radiation and convection. Further, X-ray direct observations of mold filling behaviors of cast iron were carried out in order to better understand the physical fundamentals and to evaluate simulated results. It was found that: 1) Mold filling behaviors of cast iron are quite different from those of aluminum alloy castings depending on the gating system and thickness of the casting. 2) Bubble floating-up often occurred in the case of the downward flow. 3) Although the newly developed simulation code could predict the mold filling to some extent, further work needs to be done, including better understanding of the heat transfer mechanism between the melt and pattern.