T his study was conducted to explore the effectiveness of Computer Assisted Language Learning (CALL) on the develop- ment of some English writing sub-skills (paragraph, grammar, spelling, punctuations) of ESP University students of Engineering. Participants of the current study were 38 students, divided into two groups (experimental and control), with  19 each. Being pre-tested, no significant difference existed between the groups. After the treatment (teaching & learning writing sub-skills) by using the CALL, only to the experimental group, while the regular method was applied to the control one. The findings revealed that the  experimental group in post-tests outperformed the control group in 3 CALL writing sub-skills (paragraph grammar and punctuation), whereas in the spelling skill, no considerable difference existed between the two groups.  The study suggests that CALL English writing sub-skills could be more functional and beneficial both linguistically and socially if, as much as possible, highly-prepared computer software learning programs as Computer-Assisted Writing (CAW),  and well trained English learners are more integrated