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Workers are an important resource in production activities who produce value-added products for the company. However, workers can also cause losses caused by work errors (human error). Human error has a close relationship with the level of reliability of a worker (human reliability) (Masitoh, et al, 2013). Workers with low level of reliability can cause product defects. The workload received by workers affects the operators performance. PT Wafiq Mitra Teknik has a problem that there is a product defect caused by human error at a milling work station which has three workers with workloads that require a level of precision and complex work. Disability that occurs exceeds the tolerance limit of the companys provisions, namely with an average number of disabilities of 0.012%, while the standard tolerance set is 0.002%. Therefore, this study aims to analyze the level of operator reliability and mental workload experienced by the operator at the milling work station. This research is a descriptive qualitative research using the HEART method to determine the level of reliability of workers and NASA-TLX to determine the mental workload experienced by the operator. The results of the study of mental workload with the NASA-TLX method showed that the operators mental workload had a high category for operator 1 by 78%, while operators 2 and 3 had a very high category of 80% and 82% respectively. The results of measuring the reliability level (Rm) of the operator in carrying out work on the work station milling the 1st respondent has a value of Rm = 0.4350, the second respondent has a value of Rm = 0.3627 and the 3rd respondent has a value of Rm = 0.2005 . The value of Rm <0.5 indicates a low level of reliability. This research proposes improvement of work methods by designing a good standard operating procedure, providing training and supervision (on the job training) to operators and improving work instructions that are good for companies to minimize product defects and operator mental workload. This study provides benefits for the company to determine the level of reliability and workload experienced by the operator, so that the company gives attention and supervision to the operator to be able to improve the operators reliability at work.