Communication apprehension (CA) is one important communication barrier, which can have significant negative impacts on many aspects of a person's life including perceived ability in communication, study, work performance, and promotions. On the other hand, CA has been found to be caused by many possible factors, including a lack of self-confidence and low self-esteem. This study investigated and compared CA in L1 (Thai) and CA in L2 (English) of students in a diploma program in English. The results revealed that their CA in L2 was higher than their CA in L1 in every dimension: group discussions, interpersonal conversations, meetings, and public speaking. Also, their total CA or trait-like CA in L2 was found to be higher than that in L1. In addition, when CA in L2 of the diploma students was compared to that of master's degree students with a similar major study, their CA in L2 in every dimension, as well as their trait-like CA, was found to be higher. The results may lead to a better understanding of the students in these two academic programs. Hence, the English language teaching approaches applied with these two categories of graduate students can be adjusted by instructors in order to accommodate and facilitate students who possess different types of communication traits.