Effects of day-length and GA₃ on flowering and endogenous hormone levels during flowering process of Rhynchostylis gigantea (Lindl.) Ridl. were studied. Three-year-old plants were grown under natural condition and short-day (10 h of light) combined with GA₃ at 0 and 3,000 ppm, for 3 months. The results revealed that short-day could induce flower faster than natural condition around two weeks. Short-day and natural condition had effects on stem height and bush width, while GA₃ had effects only on number of leaves per plant. ABA in leaf seemed to decreased in all treatments accept under NC-GA₃, there was a peak of ABA at 30 days. Furthermore, under SD, GA₃ seemed to decrease ABA in leaf. In shoot, ABA concentration was increased during 0-30 days in all treatments, after that GA₃ seemed to decrease ABA concentration under NC, while the ABA concentration kept constancy under SD. On the other hand, GA₃ seemed to boot t-ZR up in leaf under SD, but did not in shoot, where GA₃ seemed to drop t-ZR. The reducing ABA and increasing t-ZR in leaf and/ or shoot might be related to flower buds initiation and early flowering of R. gigantea, especially, under SD.