An Arabidopsis small G protein, RabG3b, has been previously characterized as a positive regulator of xylem development in Arabidopsis. Transgenic poplars overexpressing a constitutively active form of RabG3b (RabG3bCA) were developed, and their performance as potential biomass crops was evaluated in the field in Suwon, Korea (37 degrees 26'N, 126 degrees 98'E) from May to October 2016. RabG3bCA transgenic poplars showed increased stem growth and produced 3-4 times greater dry biomass than hybrid control poplars. Fiber length and diameter were also increased by 15-26% and 28-38%, respectively. Transgenic poplars were analyzed for enzymatic digestibility and biofuel production after chemical pretreatments. Glucose and ethanol yields were higher in pretreated transgenic poplars than hybrid control, and the pretreatment effectiveness of sodium hydroxide was better than that of the other chemicals. We propose that RabG3bCA can add value to the development of biomass crops.