This study aims to explore the effectiveness of a scanning on-screen keyboard for persons with severe motor disabilities. In order to enhance the typing performance, the on-screen keyboard employs the group-row-column scanning and a frequency-of-use layout. Besides the international alphabetical layout, the on-screen keyboard also provides two Chinese selection layouts for Chinese users. The usability evaluation was carried out with simulations and experiments. The usability evaluation was conducted to compare the performance with that of the QWERTY layout of block scanning built in Microsoft Windows XP. The results demonstrated that the performance of using the scanning on-screen keyboard was better than that of using the Windows XP. No significant difference between the error rates of the two different keyboards was found.