Abstract: This study aims to investigate the rate of decomposition and nutrient dynamics of fine root and grass root in a complex ecosystem of Alnus formosanaLolium multiflorum with one year buried bag experiment. The yearly dry weight remaining rate were 39.47%, 32.23%, 37.11%, 16.93% and 24.05% for fine root 0—1 mm, 1—2 mm, 0—2 mm, grass root and mixed root respectively. The process of the decomposition of fine root and grass root was simulated by Olson exponential equation, which showed good degree of fitting (p<0.01). At the initial phase of fine root decomposition, N and P and Ca concentrations increased, while K concentration declined adversely, and the Mg concentration had fluctuated slightly. The concentration of N and K for grass root and mixed root had the same trend as fine root during the decomposition process. However, the other nutrient concentrations demonstrated different trend. The residual rate of K, Mg and P demonstrated the same trend as dry weight remaining rate during the decomposition process of fine root and grass root. On the contrary, the residual rate of N and Ca decreased slowly. The releasing rate of K was the most quickly among all the elements, and the next were Mg, P, N and Ca lied in the last. The decomposition rate of mixed process and the releasing rate of N, P, K and Ca approximately lied between the fine root (0—2 mm) which decomposed by itself and grass root. On the contrary, the releasing rate of Mg was larger than the fine root (0—2 mm) which decomposed by itself and grass root.