The goal of this study is to propose the effective method of investigating the injurious factors and make improvement plans that prevents the workers against musculoskeletal disorders at plastic product companies and the same business field with similar working conditions and process. A questionnaire are adopted to analyze the symptoms of workers' musculoskeletal disorders, and ergonomic assessment methods such as RULA, OWAS are performed to find out harmful factors of workplace and working posture. Based on the result of the evaluation, to enhance the working environment, improvement of worktable, working space, tools, and outfit was suggested, and induction of mechanical system was also suggested. It can be concluded that the method and process described in this paper could be helpful for diagnosing the musculoskeletal disorders risk factors and making improvement plans to the plastic product companies and the same business field with similar working conditions and process.