A small color laser printer using four all-in-one color cartridges and one laser scanning unit (LSU) has been developed. Each color cartridge contains an OPC drum, a charger, a cleaner, and a color developing unit. The cartridges are rotationally switched to produce each color image which is superimposed on an intermediate transfer (IMT) belt. The superimposed full-color image is transferred to paper all at once. The primary feature of this structure is the use of four all-in-one cartridges. It makes color laser printer as easy to use as monochrome ones. When using four OPC drums, each OPC drum has mechanical differences which would normally cause registration errors among colors in conventional mechanisms. The newly developed “QuaLity Positioning Technology” (abbreviated to QLPT) mechanism cancels the registration errors and ensures better image quality. The QLPT mechanism consists of three points, which are1) Holding and locating mechanism;2) Solid coupling and synchronizing mechanism;3) Slipping correction mechanism.The essence of the QLPT mechanism is described below.