Today's smart phone users spend a considerable amount of time reading texts on mobile displays. Effective readability factors encourage more users to engage themselves into mobile internet service. An investigation through relevant literature survey uncovers that a lot of mobile internet issues are still remain unanswered. The objective of this research was to investigate the impacts of font height, display size and difference in language on the readability using the smart phone. The objective and subjective measures were used to determine the readability of both Korean and English users. Significant differences were found in the reading time and difficulty for different font heights and display sizes. In the case of Koreans, display size has more influences than font height. However, English users perceived the font height to be highly crucial in reading difficulty due to the difference in language. Guidelines of appropriate font and display sizes for full browsing tasks were discussed under relevant conditions.