The development of language skills, reading and writing, is very important for hearing impaired students. However, there is no evident about the current language proficiency of Thai hearing impaired students. Therefore, the purposes of this research were to compare the language achievement of Thai hearing impaired students with the national average scores of Thai language and to predict differences in Thai language skills when they graduate from colleges. The sample groups in this study were 218 students who are hearing impaired and enrolled in schools for the deaf in Bangkok's area. The testing score of the hearing impaired students was compared with the average score of the NIETS (National Institute of Education Testing Service). The results showed that the hearing impaired students have lower scores than national average scores about 52 to 54 percentages, which is indicated that the hearing impaired students have a very serious problem. This finding is opposite from the most Thai people thinking about the reading ability of the hearing impaired students that they can read Thai as normal students, which is incorrect. Therefore, it should be urgent to the Thai community and, perhaps, other country that it is necessary to improve learning processes and develop proper technologies for hearing impaired students to increase their language skills. (Contains 5 tables and 1 figure.)