This paper studied the root decomposition and its nutrient dynamics of Triploid populus tomentosa and Lolium multiflorum in a plantation converted from farmland at Tianquan County of Sichuan Province. The results showed that the weight loss rate of fine root and grass root showed an exponential relationship with time, with an annual loss of 76.17%, 69.80%, 73.44% and 79.53% for fine roots of 0 approximately 1, 1 approximately 2 and 0 approximately 2 mm in diameter and grass root, respectively. During the decomposition of fine roots, their N and Ca contents increased, while P, K and Mg contents were in adverse. The nutrient contents of grass root during its decomposition had an irregular variation. The remaining percentage of fine root P, K and Mg had the same trend as root weight loss during its decomposition, while that of N and Ca had a slight decrease. For grass root, its N, P, K, Ca and Mg contents declined rapidly at the initial stage of decomposition, and then decreased slowly with irregular deposition rate.