Satsuma mandarin (Citrus unshiu Marc.) is characterized by its tender peel and seedless nature. The representative cultivar of Satsuma mandarin is Miyagawa wase. Another cultivar, Ueno wase was developed from the bud mutation of a Miyagawa wase tree. Compared to Miyagawa wase fruit, Ueno wase fruit is known to have less albedo, higher sugar content, and to mature earlier. To find genes related to this bud mutation, a citrus 300 k DNA micro array was developed for gene expression analyses of the fruits and leaves of each cultivar. When up-regulated genes were classified in the Cluster of Orthologous Groups of proteins database, the number of genes included differed significantly between the two cultivars. Ten genes, including cytochrome P450, ${alpha}$-mannosidase, S-adenosylmethionine decarboxylase, carboxyesterase, anthranilate synthase, glycosyl hydrolase family-17, ethylene response factor subfamily, and alcohol dehydrogenase, showing significant differences in gene expression by DNA micro array were selected, and their expression patterns were examined by RT-PCR. The information gained from genes that are up (or down)-regulated upon bud mutation may explain the observed phenotypes, while also increasing the understanding of genes related to citrus bud mutation.