Abstract: 【Objectives】 The release character of Si form the slag-based Si fertilizer affects the efficiency of Si fertilizer, which might be governed by the cooling process and granular size of slag. So the study on the Si-release characters of slag-based Si-fertilizers will provide basis for their field application. 【Methods】 Three different types of steel slag, including S1 (in powder form, water-cooling), S2 (in granular form, water-cooling), and S3 (in granular form, air-cooling) with identically available Si content, were used for incubation trial inboth soil suspension and distilled water media at 35℃ and 25℃. Leaching solutions were collected through centrifugation at regular intervals. After centrifugation, samples were added with quantitative amount of water and incubated sequentially till the trial end.【Results】 In the soil suspension-incubation experiment, the rate of Si released from slag in the first day was mainly influenced by slag cooling process, but was impacted principally by temperature and granular size of slag as the experiment later on. The relationship between the cumulative Si release and time could be expressed with the exponential equation: y=kxm. After 97 days of soil incubation, the Si-releasing percentage (the ratio of cumulative Si release to total available Si content) of S1, S2 and S3 were 37.3%, 30.3% and 27.3%, respectively, at 35℃, and 14.3%, 7.9% and 10.2% at 25℃. In distilled water-incubation experiment, the first day release of Si was mainly influenced by temperature, afterwards principally by granular size of slag, never by slag cooling process. The relationship between the cumulative Si release and incubation time could be expressed with linear equation (y=ax+b). After 97 days incubation in water, the Si-releasing percentages of S1, S2 and S3 were 0.22%, 0.16% and 0.16%, respectively, at 35℃, and 0.17%, 0.13% and 0.14% at 25℃. In the soil suspension-incubation experiment, the pH values of leaching solutions from the soils added with steel slag were increased in the first 67 days incubation period, then their pH values were kept similar with CK at 25℃, but the pH values of treatments were always higher than that of CK throughout the whole incubation period at 35℃.In the distilled water-incubation experiment, pH and EC of S1 were significantly higher than those of S2 and S3 within the same incubation period, temperature did not show much impact on pH and EC.【Conclusions】 Si-releasing characteristic of slag was principally influenced by incubation medium and temperature, followed by particle size of slag. Si-releasing rate was significantly higher in the soil-incubation condition than in the distilled water-incubation condition. High temperature and fine particles are good for the release of Si. Therefore, steel slag is recommended to be completely pulverized before application into soil.