Parallel with the global outbreak of Covid-19 disease in 2020 which widely affected the educational milieu, many institutions of higher education shifted to fully online blended and/or synchronous courses and programs. The extent to which each of these modalities (i.e. fully online blended courses versus online real-time ones) may contribute to language learners’ achievement is largely unknown. Previous studies on blended learning (BL) across various disciplines including foreign language teaching have largely focused on courses featuring a combination of face-to-face and online sessions. This quantitative quasi-experimental study presents an attempt to compare the effectiveness of a fully online blended technical English course with a mix of online synchronous sessions and asynchronous interactive content with that of an online real-time course. The language achievement of 25 university level students attending this online blended course was compared with that of students in an online synchronous course. Drawing on independent and paired sample t-test results obtained from two sets of pre and posttests, it was observed that while both groups performed significantly better in the final language achievement test, students in the fully online synchronous course outperformed those in the blended one. The findings speak to the significance of careful design of online blended courses in terms of session and content delivery along with the essence of increasing peer-to-peer and learner-teacher interaction opportunities to improve the effectiveness of these courses for language learners.