Results from two sets of field experiments carried out during 1984 and 1985 revealed that Japanese mint(Mentha arvensisL.) was associated with both broad‐leaf and grass weeds dominant in summer and rainy seasons representing the period of first and second harvests, respectively. The weed biomass exceeded that of the crop from the very beginning of the crop cycle, and the most rapid increase in weed biomass was noted between 30 and 45 days after planting in first harvest and the first 45 days in second harvest. Unrestricted weed growth significantly reduced Japanese mint oil yield by 74.4% and 70.0%, respectively in first and second harvests. The critical period of weed interference was between 30 and 75 days in first harvest and 15 and 45 days in second harvest. Weed infestation during the critical periods caused significant reduction in plant height and herb and oil yields, and weed infestation beyond the critical periods did not have any significant influence on these parameters. The leaf:stem ratio and oil concentration in herb was also reduced significantly due to weed infestation; the oil quality in respect of menthol content, however, was not affected. Reduction in crop biomass was highly correlated with weed biomass.