AbstractWhile the use of Mobile Instant Messaging (MIM) has been expanding vastly along with mobile technology, existing MIM applications have not sufficiently addressed the needs of elderly users. MIM applications have great potential to support social interactions, and thus contribute to the well-being of the elderly. User interface design features such as small font size, confusing icons and application flow in current MIM applications, however, make them difficult for the elderly to learn and use. Our research aims to provide a usable and accessible MIM application for this user group. In this research, we combined user-centered and participatory action research principles. We conducted a focus group interview and user testing to identify the usability and accessibility issues in current MIM solutions, and collected requirements from elderly users. Based on the requirements and the principles for accessible design, we designed and developed a MIM prototype named Your IM to address the usability and accessibility issues faced by elderly when using MIM.