This study was undertaken to examine relationship between tooth size and arch dimension in dental crowding. Two groups of dental casts were selected on the basis of dental crowding. One group, consisting of 51 pairs of dental casts (24 male and 27 female), exhibited remarkable dental crowding. A second group, consisting of 60 pairs of dental casts (30 male and 30 female), exhibited little or no crowding. Mean and standard deviation of the following parameters were used to compare two groups: individual and collective mesiodistal tooth diameters, buccal and lingual arch widths and arch area. The following results were obtained. 1. The crowed group revealed larger tooth size than non crowd group. (p<0.01) 2. The crowded group smaller maxillary dental arch dimension than noncrowded group except lingual arch width at canine region. (p<0.01) 3. The crowded group revealed smaller mandibular dental arch dimension than noncrowded group except lingal arch width at second premolar region in the male and buccal arch width at canine, premolar region in the female. (p<0.01) 4. The crowded group revealed smaller arch area than noncrowded group in the female (p<0.01), but there was no significance in the male.