A cellular manufacturing system is a kind of lean and flexible manufacturing systems in which a worker or a group of workers carry out all assembly processes of a product. Assembly times of workers are generally improved in an exponential manner by repeating same assembly processes. However, the workers in the cellular manufacturing system are not provided with enough time to learn the assembly processes, since product mixes and production volumes are changeable in a short period of time. The objective of this study is to propose a work instruction system for untrained workers in assembly cells to understand the assembly processes quickly and reduce their assembly times of products without repeating same assembly processes. This study firstly records workers' assembly processes by using video equipment and analyzes the learning processes of the workers in order to propose an effective strategy for the workers to reduce the assembly processes in the cellular manufacturing system. According to the proposed strategy, a prototype of a work instruction system is developed for untrained workers in assembly cells. The prototype system provides graphical user interfaces explaining the information of assembly processes for the workers to facilitate understanding the assembly processes. Some experiments are carried out for assembling same toy cars built with Lego blocks in order to evaluate the effectiveness of the developed work instruction system. Experimental results of the proposed work instruction system are compared with the ones of a simple work instruction system. Through the comparison, it is recognized that the proposed work instruction system is superior to the simple work instruction system from the viewpoint of the reduction of assembly times. At the last part of this study, we propose a prediction method of assembly times of workers by measuring biological information as a heart rate, in order to carry out dynamic production management through the work instruction system.