This study investigated the similarities and differences in the use of discourse connectors (DCs) in argumentative essays of American undergraduate students (AMs), Thai with high-English exposure (THHs) and Thai with low-English exposure (THLs). The data of these three groups were collected from 60 essays; 20 essays were from the corpus of University of Michigan with a total of 43 essays, the 40 Thai data were selected from 300 Thai university students based on their English exposure scores. Adopting the theoretical framework of Halliday and Hasan (1976), Biber et al. (1999), and Cowan (2008), there were five categories of DCs in this study: Additive, Adversative, Causal, Temporal, and Continuatives. The data were statistically analyzed in terms of mean, standard deviation, t-test, and One-Way ANOVA. It was found that there was a significant difference in two categories: Causal and Temporal. The t-test for Causal was 0.007, and the t-test for Temporal was 0.005 (p < 0.05). The differences in the use of DCs in AMs and THHs and THLs could be the effect of interlanguage processes, i.e., Language Transfer, Transfer of training, and Strategies of second language communication. Additive category was most frequently used by all three groups, especially the use of the DC lexis "and." It is interesting to discover that pragmatically speaking "and" represents many discourse functions beside Addition. It is used in all main categories, i.e., Adversative, Causal, Temporal and Continuatives.