H orizontal Directional Drilling (HDD) is a drilling technique without damagingthe ground surface. The HDD process involves stages that require highconcentration, physical skills, and is often carried out in challenging workingconditions, causing mental workload and fatigue for HDD workers. HDD must becarried out through real-time monitoring which requires workers to get resultswithout errors so that workers experience mental workload pressure and fatigue.The aim of this research is to measure and minimize the level of mental workloadand fatigue in HDD workers and determine proposed improvements based on thewhy why diagram. Measuring mental workload using the RSME method and fatigueusing SOFI and why why diagrams to look for causes and effects in the mentalworkload and fatigue experienced by HDD workers. Based on the RSME analysis,the scale value obtained based on 6 indicators of Work Load, Work Difficulty,Work Performance, Work Mental Effort, Work Anxiety and Work Fatigue is 92.05which is in the category of very large effort and the KiK indicator or work fatiguegets a scale value. The highest was 106.52 with the business category given beingso large that HDD workers experienced mental workload. Based on the SOFIresults, the sleepiness dimension received a score of 21.83 in the high fatigue levelcategory. So the proposed improvements to the cause and effect results of the whywhy diagram are additional workforce, adjusting work shifts, and monitoring andevaluating HDD workers.Keywords :RSME, SOFI, mental workload, fatigue, Horizontal DirectionalDrilling.