Tactile dots (dot-shaped tactile symbols) placed on the operation keys of consumer products such as cellular phones contribute to improving accessibility for everyone, including persons with visual impairment. JISC (Japanese Industrial Standards Committee) and ISO (International Organization for Standardization) standardized tactile dots and bars. However, sufficient reliable data on the appropriate sizes and cross-section shapes of tactile dots and bars was not necessarily available. Therefore, more quantitative data for optimal dimensions of tactile dots and bars is required to revise existing standards and devise new standards. In this paper, we evaluated influences of the height (0.1, 0.3, 0.55 and 0.75 mm, without tactile dot) and the tip radius of curvature (0.1, 0.3, 0.5, 0.7 and 0.9 mm) of tactile dots on the operational performance in cellular phones. 16 sighted younger participants, whose hand was covered by a curtain, were asked to operate cellular phones with a tactile dot on its key 5 and without tactile dots. As the result, both too high and low height dots are not effective to improve the operational performance of cellular phones and there is an appropriate range of the height of tactile dots regardless of the tip radius of curvature. Furthermore, participants performed better at a particular height with larger tip radius of curvature.