Nitrogen (N) is the growth-limiting nutrient of plants in terrestrial ecosystems,especially in grassland ecosystems. The availability of N is determined by N cycling processes in terrestrial ecosystems. To better understand the N cycles, a key way is to exactly determine the rates of N transformation. To date,~(15)N stable isotope technique is highly recommended for quantifying the processes of N transformation. The isotope technique is generally classified into four categories as ~(15)N natural abundance,~(15)N reduction, ~(15)N pool dilution and ~(15)N tracer techniques. In this paper, the progresses of researches on N transformation using ~(15) N technique with a focus of ~(15) N pool dilution and ~(15) N tracer techniques in grassland ecosystems were summarized. Firstly, two detailed protocols were summarized from published papers for ~(15)N pool dilution and ~(15)N tracer techniques, respectively. Secondly, the recent findings of N transformation in response to human activities (fertilization,grazing, fire and mowing) and global change (global warming, increasing precipitation, atmospheric N deposition and increased CO_2 in atmosphere) using the above two N isotope techniques in natural grassland ecosystems were reviewed. The isotope techniques described here were also applicable to other terrestrial ecosystems, such as forest ecosystems and agro-ecosystems.