This paper focuses on small touch key size and spacing that affect the usability of the smart phones. The experiments examined thirty-six different touch key designs, combinations of four touch key sizes (width (mm) ${ imes}$height (mm); $4{ imes}4$, $4{ imes}7$, $7{ imes}4$, and $7{ imes}7$), and nine spacings (horizontal (mm)${ imes}$vertical (mm); $0{ imes}0$, $0{ imes}1$, $0{ imes}3$, $1{ imes}0$, $1{ imes}1$, $1{ imes}3$, $3{ imes}0$, $3{ imes}1$, and $3{ imes}3$). Forty participants volunteered in the experiment. The completion time, error rate, identification discomfort, and control discomfort were measured. The results revealed that the touch key sizes of $7{ imes}7$ and $7{ imes}4$ provide the best performance in terms of the error rate measure, while the touch key size of $7{ imes}7$ provides the best results for other measures. In addition, touch key spacings of $1{ imes}1$, $1{ imes}3$ and $3{ imes}1$ are regarded as the best for all the measures. The results of this study suggest a practical approach for the small touch key design for the smart phones.