Abstract Background: Athletes have used herbs such as ginseng, caffeine, and ephedrine to improve exercise and sports performance. A popular herb among Southeast Asian people is Eurycoma longifolia Jack (ElJ) that is purportedly important for health, and even recovery and fitness after an intense training. However, its use for training in athletes was unclear. At such, this study targeted to explore the outcome of consuming ElJ supplementation when implementing supramaximal eccentric training on strength, hormones and muscle thickness in athletes. Methods: Sixteen rugby 7s athletes (age: 21.5±1.5y, height: 1.74±0.06m, weight: 80.0±10.0kg) evenly assigned to one of two groups that supplemented with either ElJ (400mg per day) or similar taste and appearance placebo (PLA). They performed a supramaximal eccentric training method three times per week for 3 weeks in which the rising the load every week from 110, 120, and 130% of concentric one repetition maximum (1RM). Dependent variables of 1RM concentric and 1RM eccentric strength during leg press; salivary hormone (testosterone and cortisol); and diameter of the muscles (vastus medialis, vastus lateralis, and rectus femoris) were collected within a week prior to, and after the training period. Results: Mixed ANOVA showed a significant interaction of testosterone (p=0.018), 1RM concentric, 1RM eccentric, muscle thicknesses of vastus medialis, vastus lateralis, and rectus femoris have produced significant changes (p<0.05) over time. However, none was significant (p>0.05)for between-group comparison. Conclusion:The study concluded that improvements in muscular strength appears significant after three weeks of supramaximal eccentric training, either with or without ElJ. Furthermore, increases in muscle thickness and changes in hormones occurred similarly across groups for the highly trained cohort in the present study.