The purpose of this research paper is to study the grammar errors in conversation of Thai Japanese Learners. Data were collected from 30 samples of basic Japanese language learners by interviewing. The result of the research showed that Thai Japanese learners have errors in using verb, particles and adjectives. The errors in using verbs was found in many types such as verb conjugation by tense, using verb was not consistent with the question and omission of verbs in sentences. The particles that Thai Japanese learners always use wrongly were the particle that indicate parties that action toward to and the particle that indicate means and method. The most error in using adjectives is same as the use of verbs that was the conjugation by tense. Moreover, there was errors in expressing opinion and comparisons.