T he aim of this study was to develop collaborative writing using the Whats Apps among 3^rd year English majors at Misr University for Science and Technology (MUST) in Fall 2017 semester. The problem of the study was that 3rd year English majors were not motivated enough in the writing class and had difficulty in communicating through writing with each other during the limited time of the class. So, the idea of using Whats  Apps for developing writing skills was necessary especially that there is Wi Fi available at MUST. The researcher used the quasi- experimental design with two groups, experimental and control. Two instruments were used; a pre/ post collaborative writing skills test, and a reflection sheet. The results of the reflection sheet showed that 3rd year English majors did not use the Whats App in learning any language skill before and that they were satisfied with using it in the writing class. The writing skills test revealed that     using Whats App was effective in developing collaborative writing among the participants in the study.  Also, the results of the reflection sheet proved that the students of the experimental group were satisfied with using Whats App for developing collaborative writing.