An experimental study is conducted to investigate the smolder initiation behavior of horizontally oriented flexible Polyurethane foams under natural convection. This study mainly aims to explore the controlling factors affecting the smolder initiation time of polyurethane foams under natural convection. Different ignition powers are tested for the investigation of its influence on the smolder initiation time. It is found that the smolder initiation time decreases with the ignition power. In order to shed some insight on the effect of retardant additives on the smolder initiation, both fire retarded and non-fire retarded polyurethane foams are addressed for comparison. The experimental results suggest that the smolder initiation time of the fire retarded foams is longer than that of the non-fire retarded foams at a higher ignition power. At a lower ignition power, an inverse result is observed. Besides the ignition power and the retardant additives that show effects on the smolder initiation of the polyurethane foam, it is also found that the smolder initiation time increases with the foam length.