This study was done to determine the effects of bale sizes of bio-ammoniated rice straw on its nutrient quality and in vitro digestibility. Rice straw were bio-ammoniated by adding 2 g urea and 1 g probiotic/kg DM. By following a completely randomized design, rice straw was baled in 3 different weights (15, 25, and 35 kg) with six replications for each treatment, and then stored for 3 weeks. In the end of the week 3, bales were opened, aired, and then sampled for proximate analysis (dry matter, organic matter, crude protein, and crude fiber) and digestibility by in vitro gas production method. Using analysis of variance method, the proximate data showed that greater the bales size (15, 25, and 35 kg) increased crude protein (7.59, 7.86, and 9.95%, respectively; P