The purpose of this study was to reveal the optimum size and inter-key spacing of numeric square keys for touch screens. Six male students (22-25 years old) and three female students (21-24 years old) volunteered as subjects for this experiment. Each subject took part in data entry tasks using numeric square keys of touch devices. The sizes of keys were 6, 12, 21, 30 and 39 mm and each the inter-key spacing was 0, 3, 6, 12 and 21 mm. Response times with key sizes of 6 and 12 mm were significantly slower than with key sizes of 21 and 30 mm (p < 0.001). Furthermore, the key size of 6 mm significantly caused more errors than the key sizes of 12, 21, 30 and 39 mm (p < 0.05). The response time with inter-key spacing of 3 mm was significantly faster than with that of 0, 6, 12 and 21 mm (p < 0.001). Inter-key spacing of 0 mm significantly produced more errors than other inter-key spacing. Subjective ratings for inter-key spacing of 3, 6 and 12 mm were significantly better than those of 0 and 21 mm (p < 0.05). These results suggested that the optimum size of numeric square keys for touch screens should be more than 21 mm and optimum inter-key spacing should be from 3 to 6 mm. Optimum key size, however, must be selected with regard to the limitation of screen size.