Rabatak Inscription was excavated from Baghlan in Afghan in 1993, which recorded the first four Kushan Kings' names, and ascertained the ancestry of these four kings. Therefore, the discovery of the first four Kushan Kings, which is very significant in Kushana study, established the early Kushana history for the first time. Meanwhile, the problem of Kanishka era comes to a new evidence. Significantly, the record rule in the new inscription suggested the relationship between the Unknow Era used in Vima Takto period and the Arsacid Era. This discovery makes a clue for the King Kanishka's Era which was named after him.