Flax (Linum usitatissimum L.) has been used for the edible oil in Korea. The evaluate the effect of plant growth retardants on flaxseed yield, oil content, and gibberellin level, chloromequat chloride (CMC), paclobutrazol (PBZ), and prohexadione-Ca (PHC) were used as plant growth retardants in this study. These plant growth retardants were foliar-sprayed to flax plant at 55 days after seeding. The concentrations of each plant growth retardant were as followed: CMC 250, 500, 1000 ppm, PBZ 40, 80, 160 ppm and PHC 500, 1000, 2000 ppm. PHC treatment to forming bolls was more stimulated than CMC and PBZ. The highest ripened seed rate was observed in PHC treatment at 2000ppm. The high see yields were obtained in PHC treatment following PBZ and CMC, in turn. Seed yield that significantly increased in PBZ and PHC was found to be increased 12.4 ~ 23.9% as compared to the control. The PHC showed higher flaxseed production and oil yield than that of CMC and PBZ. The results obtained in the present study suggest that higher concentration of plant growth retardant (PHC) increased flaxseed yield and oil content. The optimal concentration of PHC treatment was observed in 2000 ppm. It concludes that the foliar application of PHC 2000 ppm may be useful for the increasing oil and higher seed production in flax plants.