To develop the effective methods of flowering stimulation, Hinoki cypress (Chamaecyparis obtusa Sieh. et Zuec.) grafts growing in a seed orchard and in a clone bank in Southern Breeding Station of Cheju were applied with gibberellin treatments, and predicted the seed production potential, In the seed orchard, GA_(4/7) 1.5cc was injected into the stem of drafts and sprayed whale tree crown with GA₃ 300ppm and GP_(4/7) 300ppm. Un the other hand, in the clonal archives, drafts were given intrusion of GA_(4/7) 1.5cc into the excised and open inner part of bark wind GA₃ 20㎎ and sprayed with GA₃ 300ppm. Additionally, grafts growing in the seed orchard were treated with gibberellins at 3 different periods of time and 3 different treatments during the growing season. The results obtained here are summarized as follows : 1. All of the applications of Gilbberellin promoted female flower formation. Among these, the treatment of intrusion of GA_(4/7) 1.5c into the excised and open inner part of bark was racist effective, followed by the spraying of GA₃ 300ppm. Similarly, the applications of gibberellin promoted male flower formation. 2. Regarding the time of applications, treatment on August 15 was more effective than those of August 31 and September 11 in the stimulation of female flowers. On the contrary, there was no significant difference in the number of male flowers among 3 different time treatments. 3. It was supposed that the application of the intrusion of GA_(4/7) 1.5cc into the excised and open inner part of bark on August 15 showing the best effect in female flower formation can produce 22.12㎏ seeds per ha. 4. Considerable significant difference existed among clones for both female and male flower formations. 5. Flower formation, especially female flower formation, seemed to be partially associated with the genetic potential of individual trees.