This study used a multiple-probe design across three participants to test the effectiveness of a handwriting intervention for fifth graders (age 10-11) displaying less handwriting fluency than their peers, but without spelling disorders. The 5-h handwriting intervention provided students with explicit instruction and intensive practice in writing cursive letters, words, and sentences, through fast-paced alphabet and copying activities. Intervention effects were examined on handwriting fluency, written composition (i.e., text length, clause extension, and story elements), and self-efficacy beliefs. Results showed that the handwriting intervention was highly effective in increasing students' handwriting fluency. There were also improvements in written composition in terms of clause extension and number of story elements. After the intervention, students also reported strengthened self-efficacy beliefs for grammar and usage skills. Overall, this study showed that handwriting interventions can effectively help students with limited handwriting skills to become fluent handwriters. Critically, findings are in line with the proposition that achieving handwriting fluency is important to support the development of writing.