A s an important cast alloy, hypereutectic Al-Si alloy has a low expansion coefficient and density, good cast performance and high wear resistance. However, with the increasing of silicon content, a large number of coarse and irregular lumps of primary silicon and needle-like eutectic silicon can be observed, which seriously decreases the comprehensive performance of the alloy. Therefore, primary silicon must be refined to achieve the fine dispersive Si phase with desirable properties [1,2] . It was found that adding modifier is the most popular and effective method to refine the primary silicon [3][4][5] . The effective refinement of primary Si can be obtained through the addition of some modifiers, such as P [4][5][6] , rare earth (RE) [7,8] , etc. However, the use of P can result in serious air pollution, and the costly RE used to refine primary Si is poor effective. So, researchers have been looking for new and more efficient and effective modification agents for Al-Si casting alloys.According to research [9] , Al-ZnS master alloy was used as a new refiner, and the primary silicon could be refined from more than 80 μm to 20 μm. Fan et al. also researched the modification performance of ZnS on primary Si in hypereutectic Al-Si alloy and came to the same conclusion [10] , in which ZnS was synthesized by chemical reaction using Zn and Na 2 S. Nevertheless, Abstract: A new modifying agent, ZnS, was used as a refiner to modify primary silicon in hypereutectic AlSi alloy. The factors affecting the modification results, including addition level of ZnS and holding time, were investigated. The results showed that the average size of the most effectively modified primary silicon was 28.5 μm when the ZnS mixed powder addition was 0.15wt.% with a holding time of 10 min. More important, the average size of primary silicon could remain below 40 μm despite the holding time extending to 120 min, which means ZnS is a promising modifying agent of primary Si in industrial applications. Na 2 S is so deliquescent in the air that it is not favorable for storage. The ZnS synthesized by direct chemical reaction of Zn and S can be stored trouble-free, and the yield was very high, but research concerning the refinement and modification behavior of ZnS on Al-Si alloy hardly can be found. To investigate the recession degree of ZnS modification synthesized by Zn and S and the optimal addition level, the effects of ZnS addition amount and holding time on Al-Si alloy were investigated. Key words: Experimental procedureAccording to the study of hypereutectic Al-Si alloy, the shrinkage of Al-Si alloy was the lowest when the content of silicon was between 22wt.%-25wt.% [11] .Therefore, Al-24wt.% Si alloy was chosen to be the base alloy in this study. Figure 1 shows the microstructure of Al-24Si alloy without ZnS addition. Primary silicon particles exhibit coarse irregular and star-shaped morphologies in the as-cast alloy, which will drastically degrade the mechanical properties of the castings, and the average size of primary Si is up to abo...