The article presents the experience of developing computer ontology as one of the tools for Tibetan idioms processing. A computer ontology that contains a consistent specification of meanings of lexical units with different relations between them represents a model of lexical semantics and both syntactic and semantic valencies, reflecting the Tibetan linguistic picture of the world. The article presents an attempt to classify Tibetan idioms, including compounds, which are idiomatized clips of syntactic groups that have frozen inner syntactic relations and are often characterized by omission of grammatical morphemes; and the application of this classification for idioms processing in computer ontology. The article also proposes methods of using computer ontology for avoiding idioms processing ambiguity.