Abstract: In order to study the growth mechanism of bamboo culm, and explore the methods of regulating bamboo fiber quality, different mass concentration(0, 20, 40, 60 mg/L)of gibberellin(GA₃)were sprayed on moso bamboo seedlings, and the neonatal tiller number, culm form and fiber quality were measured after treatments. Results showed that the culm height, internode length and leaf number of neonatal tillers increased significantly, the culm height of neonatal tiller under 20, 40 and 60 mg/L GA₃ treatments were 0.56, 1.19 and 1.21 times higher respectively than that under the control(0 mg/L). The mean internode length increased 0.49, 1.05 and 1.10 times, and the number of leaves increased 35.61%, 48.35% and 38.68%, respectively. Nevertheless, no significant difference was presented among treatments on the neonatal tiller number, node number and the ground diameter. With the changes of culm form, the fiber quality of neonatal tiller also varied. The length of fiber cell under the 20, 40 and 60 mg/L GA₃ treatments all had been increased significantly, 13.17%,20.68% and 31.46% longer than that under the control, respectively. However, the effects of each treatment on fiber cell width were not obvious, resulting in increasing of the fiber aspect ratio which is conducive to the fiber quality improvement. Besides, the effects of GA₃ treatment on fiber cell wall thickness, lumen diameter and fiber tissue proportion were not obvious. Research showed that the use of GA₃ could significantly improve the fiber quality of moso bamboo seedlings neonatal tiller, but couldn't significantly improve fiber production.