Recently, the government of Mongolia has decided to expand usage of Mongolian traditional script. Official letters, official certificates have to been written in Mongolian traditional script. Even if, traditional Mongolian script became taught in secondary schools and national programs implemented to teach traditional Mongolian script to public, traditional Mongolian script is still not familiar to Mongolians. In this paper, we propose automatic conversion system Mongolian Cyrillic script into Mongolian traditional script. Because of word sense disambiguation problem we need a Mongolian slot grammar parser. Mongolian lexical system is implemented using Protégé Web Ontology Language where we can describe relation between words such as: same rooted words like “шийдвэр-discussion”, “шийд-discuss”. There is no similar system implemented by Mongolians, except the system made by Inner Mongolians in China, and our system is not based on a database but based on an ontology.