The rapid development of high technology and the Internet has made potential contributions to English language education for the past few decades. Many linguistic experts have conducted relatively intriguing research on online English writings abroad. Still, there have not been many studies of online English writings in the Vietnamese higher education context so far. This study investigates cohesive devices in one hundred and thirty-five English writings made by EFL students as taking blended-learning or online English writing courses at several universities in Viet Nam for the last five years. It finds three among four types of conjunctions (including coordinating, subordinating, and adverbial conjunctions) used in these online writings remarkably. They all tend to increase in terms of the typed-word quantity and frequency in the second writing version because the initial writing samples are shorter and have fewer complex sentences than the second ones. Besides, a few conjunctions (i.e., so that, because, or although) are sometimes used ungrammatically. Finally, it reveals that the writing topics and the student's English level partly influence using cohesive devices in their online English writings. The more specialized the topic and the higher the learner's English level, the more conjunctions appear and are used harmoniously in the students' online English writings.