Background and Objective: The rapid emergence of technology in education contexts may lead many ESP teachers to integrate the new technology into their classroom. This study aimed to investigate the effects of online learning, traditional learning and blended learning on university students' achievement in an ESP course, taking field of study and gender into account as well.

Materials and Methods: The subjects of this study included 94 male and female, (62 female and 32 male), undergraduate Iranian students from Zanjan University of Medical Sciences in two fields of study: pharmacy and medicine. 61 of the subjects were students of medicine and 33 were pharmacy students in the age range of 18 - 21. Pretest and posttest were used to compare population mean scores for the learning gain before and after the course delivery in each group. To determine if there is a difference in students' achievement using the three modes of delivery, students' final exam scores were compared.

Results: The study results indicated that in traditional group, students scored higher than the online and blended groups. This difference between traditional and online group was significant. Also findings showed that students' field of study and gender did not have any significant effect on their achievement.

Conclusion: It can be concluded that traditional learning was the most effective mode in students' achievement, blended learning was the second effective mode and online learning was the least effective one.