This thesis proposes a new approach to improve the performance of Tamil speech recognition using language models. The main contribution of this thesis is the development of language models to capture co-occurrence patterns of partially free word order languages like Tamil. The models designed used sub-word units such as phonemes, syllables and morphemes as basic components of the language model. In addition the thesis explains the use of various language models at different levels of error correction. Language models based on different sub-word units, sub-word unit features and contexts...