The present study aimed to investigate the extent to which a CLIL (Content and Language Integrated Learning) course at university level in Thailand improves undergraduate Agricultural students' writing ability, agricultural content, and cultural knowledge. The study sample consisted of 27 students majoring in Agriculture at a public university in Thailand. Data were collected in the forms of writing, agricultural content, and cultural knowledge pre- and post-tests. The study findings suggested that after attending the CLIL course, the students showed significant improvement in their English writing abilities in both of the two text types: process and cause and effects, with better performance in all five aspects of writing, namely, content, organization, vocabulary, language use, and mechanics of writing. It was also found in this study that the students' agricultural content and cultural knowledge increased, and that they had positive attitudes toward the CLIL course offered to them.