A conversion method converting characters from a source character set to a destination character set. An index table, a key table, and an encoding table are provided. Source character code of a character is divided into a first part and a second part. A head offset and a tail offset are acquired in the index table according to the first part. The second part is searched in the key table based on the head offset and the tail offset. Destination character code of the character is retrieved from the encoding table matching to the searching result.