In this paper the effect of the dimensions of the polymer sheets upon the downward flame spread is studied by theoretical and experimental methods. In the theoretical method, the effect of the solid fuel dimensions on the energy balance in the solid phase is studied by the order of magnitude of parameters. In the experimental method a sample of polymethyl methacrylate (PMMA) sheets with various thicknesses (1.75-7 mm) and width (10-150 mm) is examined in a big room with quiescent air. A slit burner, a camera, a load cell and a thermocouple are used to ignite, videotape the movement of surface, record the mass of the specimen and measure the temperature during the process, respectively. In this study, theoretical model shows for a large value of the ratio of the width of the sample to its thickness, can be the processes considered by one-dimensional model. So that the temperature distribution and consequently the flame spread rate are functions of thickness and physical properties of sheet. Also, the experimental results confirm the result of theoretical model. The experimental result shows the flame spread rate is independent of the width of the sample if the ratio is bigger than ten.

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