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)×height (mm); 4×4, 4×7, 7×4, and 7×7), and nine spacings (horizontal (mm)×vertical (mm); 0×0, 0×1, 0×3, 1×0, 1×1, 1×3, 3×0, 3×1, and 3×3). 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 7×7 and 7×4 provide the best performance in terms of the error rate measure, while the touch key size of 7×7 provides the best results for other measures. In addition, touch key spacings of 1×1, 1×3 and 3×1 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.