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 NC had effects on stem height and bush width, while GA3 had effects only on number of leaves per plant. ABA in leaf seemed to decrease in all treatments accept under NC+GA3 at 0 ppm, there was a peak of ABA at 30 days after treatment. Furthermore, under SD, GA3 seemed to decrease ABA in leaf. In shoot, ABA level was increased during 0-30 days after treatment in all treatments, after that GA3 seemed to decrease ABA level under NC, while the ABA level 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.

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