The objectives of the study are to develop a curriculum for an English training for counter clerks in convenience stores in Chiang Mai, Thailand and to evaluate the effectiveness of the developed curriculum. A mixed-method research design was employed. In the curriculum development phase, participants were 20 counter clerks and 10 administrators while 40 counter clerks and 271 English-speaking customers participated in the curriculum evaluation phase. Findings revealed nine tasks of a counter clerk which require English. Trainees' English communicative competence improved significantly from the pretest to the posttest and from the pretest to the delayed posttest. However, there was a decrease of the mean scores at the delayed posttest when compared with the posttest for all areas of communicative competence. English-speaking customers were highly satisfied with trainees' use of English, and trainees perceived that the curriculum was very efficient. The study suggested that an English curriculum for adult learners must reflect a wide range of real-life tasks and incorporate real-world language functions.