Previous work has demonstrated upward flame spread on vertical surfaces to be one of the most hazardous fire scenarios. To assess the risk of this scenario, several models have been developed to predict the flame spread rate, relying on empirical correlations of flame height and heat feedback to unburned surface ahead of pyrolysis region. However, the width effect was not regarded particularly in those models but to influence flame thickness, causing the variation of radiation. Therefore, experiment has been designed to access the width effect. Samples used were 6 and 20 mm thick clear PMMA with height of 1000 mm and widths of 100, 300, 500, 700 and 900 mm. Our data showed that the width effect was significant for samples less than 300 mm wide and not significant for 300 to 900 mm wide samples. In addition, the width effect was slight in total heat flux distribution and not obvious in flame height correlation. As to the radiant heat flux distribution, our measurements were much lower than recognized in previous studies.