An aim of this research is to grasp related words of unknown word. Currently, several lexical dictionaries have been developed for semantic retrieval such as WordNet and FrameNet. However, more new words are created in every day because of new trends, new paradigm, new technology, etc. And, it is impossible to contain all of these new words. The existing methods, which grasp the meaning of unknown word, have a limitation that is not exact. To solve this limitation, we have studied the way how to make relations between known words and unknown word. As a result, we found a noble method using co-occurrence, wordnet and Bayesian probability. The method could find what words are related with unknown word and how much weight other words relate with unknown word.