This study aimed at investigating natural thumb zone of the user in smartphone touchscreen with one-handed grip. The subjects were divided into three groups according to their thumb lengths: small, medium, large. Experiments were carried out using three smartphones of different sizes: 4.7 in., 5.1 in., 5.2 in. The natural thumb zone of each group was analyzed using heat-map method. In large group, the thumb zone occupied more than 50% of the touch screen, while small and medium groups occupied only about 30% in all smartphones of different sizes. Compare to other groups, the thumb zone ratio of the small group in 5.2 in. smartphone was found greatly reduced. The lowermost region and the upper-left region of the smartphone touchscreen are not commonly included in natural thumb zone, regardless of the thumb-length groups. The results of this study could be used for designing new mobile device to enhance the usability of one-handed interaction; small devices with a touch screen as well.