Information overload is a problem in the Information Age and Information visualization is an approach to provide an overview of the content of a web site. Tag cloud is one of the ways to represent information as an image of a group of words. However, there are limitations on tag cloud generation, and one of them is due to the characteristics for the language. In order to extract tags or words for tag cloud, word segmentation is required. This paper proposes a Thai word segmentation approach for the visualization of Thai Web sites. The proposed Thai word segmentation technique is based on the longest matching technique together with a refined corpus. The results of Thai word segmentation are compatible with the results from previous BEST's contests in Thailand.