A series of lab-scale experiments were carried out for expandable polystyrene (EPS) with the densities of 18 and 25 kg/m3 and widths of 4, 8, 12 and 16 cm respectively. The representative parameters such as the length of pool fire zone, flame height and flame spread speed, were investigated. The results showed that the flame structures of EPS during flame spread process included the typical type, the reverse type and merged type. When the width of the sample was 8 cm, the flame structures behaved almost as the reverse and merged types for the worse instability of flame during spread. The average length of pool fire wais longer for 18 kg/m3 than for 25 kg/m3 with the same width, which can be attributed to the faster flame spread speed with the lower density for EPS. The flame spread speed of EPS with the density of 18 kg/m3 was almost two times of that with the density of 25 kg/m3, due to the air in the cells which promotes combustion. It was also found that the surface flame height was mostly controlled by the width of EPS, while the flame height in the pool fire was mostly controlled by the ratio of the length to the width of the pool fire.