Background: Augmentative and alternative communication (AAC) is the approach that enhances communication competence in individuals with complex communication needs. With the advancement of technology, there are varieties of AAC applications with colored-graphic symbols and speech output, improving communication’s intelligibility compared to low-tech AAC systems. However, those AAC applications had some features that were not entirely suitable for Thai users, such as symbol appearance, speech intonation, etc. Objective: This study aimed to develop the first version of the Thai AAC application, A-Speak, based on Thai culture, lexicon, and intonation and remove other constraints that other AAC applications had, such as variation in voice-output age and gender. The proficiency of A-Speak regarding communication functions was also examined. Materials and methods: The participants comprised 15 individuals with cerebral palsy and complex communication needs. The participants were trained to use the A-Speak application, installed on a tablet, to communicate. The training procedures consisted of 3 phases: Phase 1: Train to select icons; Phase 2: Shift to different categories; and Phase 3: Use A-Speak to communicate. The researchers trained the participants to achieve adequate operational skills (i.e., Phases 1 and 2) before beginning Phase 3. In Phases 1 and 2, switches were employed to facilitate participants with limited mobility to operate A-Speak by finger. The researchers also taught the participants’ caregivers to continue training them at home. The researchers collected the participants’ communication abilities regarding communication functions in the recorded form. The data was reported into code numbers according to communication proficiency. Results: After receiving A-Speak training, all participants showed improvement in their communication abilities across a variety of communication functions. Participants showed significant progress in 10 out of 12 communication functions. The communication function in which participants exhibited the most improvement was explaining skills, whereas the communication function that showed the least development was storytelling skills. Conclusion: A-Speak AAC application reduced the constraints that possibly influenced communication intelligibility in the Thai language. Nevertheless, A-Speak still had a few drawbacks that required to be corrected to increase the productivity of this program. The findings indicated that participants gained communication skills through A-Speak as a means of communication. Keywords: A-Speak, augmentative and alternative communication, application, High-tech AAC