In traditional production assembly lines, conveyors are widely used to move components from one workstation to another. The conveyor belt must stop and stay at each workstation for a specific period of time and wait until activated. Then the assembly operation is complete before the conveyor can move the components to the other workstation. Conveyor downtime accumulates and lengthens the entire assembly process. This causes a decrease in overall production efficiency, especially in automated assembly lines. This study presents a method that can increase the productivity of an assembly line by assembling moving objects on a conveyor belt using image processing and manipulators. One monocular camera is attached at the end-effector of 6DOF robot. It will apply the image processing algorithm to detect objects to be assembled on the conveyor belt. After that, the position and angle of the object are calculated and transformed to robot coordinate through transformation matrix obtained from the camera calibration process. The assembly process will be performed based on the calculated position and direction of the object and the speed of the conveyor belt. Through experiments, the system proves to have high reliability and stability and meets the requirements of industrial assembly. The system is also easy to install and maintain.