A dual key bi-step lock having a shell and a cylindrical plug in the shell. The plug has a primary keyway and a secondary keyway and at least one set of primary pin stack and at least one set of secondary pin stack between the shell and the plug, respectively coupling with the primary and the secondary keys. a longitudinal stage is formed on a cylindrical surface of the plug. An involute side face extending from the stage to the cylindrical surface of the plug. The primary key is able to dive the plug to a position of first step open at which the stage is stopped by the key pin of the secondary pin stack. The secondary pin stack is unlocked by the secondary key and the plug is able to be further driven to a position of second step open by the primary and the secondary keys from the first step open.