A modular tool system includes a plate for carrying a tool, a top plate and a base plate. A pair of locking members is disposed at two opposite ends of the tool-carrying plate. Each lock member is rotatably mounted in the tool-carrying plate, and has an axis of rotation perpendicular the tool-carrying plate. Each locking member has a keyway for receiving and locking a corresponding key of the top plate, and a key for inserting and locking in a corresponding keyway of the base plate. Turning of the keys of the top plate relative to the keyways of the tool-carrying plate locks the top plate and the tool-carrying plate together, and further turning of the keys of the top plate turns the keys of the tool-carrying plate relative to the keyways of the base plate and locks the tool-carrying plate and the base plate together.