Abstract: In the process of gasification for different size of coal particles, there are remarkable differences in the cracking mode, behavior of volatile removal and coke-slag interaction. These differences lead to the discrepancies in structural characteristics and reaction behavior for fine slag. Therefore, it is considered that the study on relationship between structure, properties and size distribution of fine slag from entrained flow gasification can provide vital guidance for analyzing the formation mechanism of fine slag and optimizing the size of coal particles for gasification. For this purpose, the fine slag from Ningdong typical GSP technology in Ningxia Province was selected as a raw material. After drying, crushing and sieving, three kinds of samples with size of <0.125, 0.125–0.250 and >0.250 mm were prepared, and called small, medium and large size samples respectively. The nitrogen adsorption, XRD, Raman spectroscopy and TGA were applied to clarify the physicochemical structure and combustion reactivity of samples. It is found that there are huge differences in the composition, structure or reactivity of the samples in different size. Precisely, three types of samples account for 22%, 46%, and 32% respectively. All the fine slag contains a large number of spherical particles and irregular particles. The sample with the middle size particles, which has the most content of residual carbon (19%) and the lowest graphitization degree (30%), shows the slightest gasification degree. It also presents the largest specific surface area (87.8 m²/g), and the optimal combustibility index regardless of the heating rate. While, the above properties of the sample with large size particles are completely opposite. Apparently, coal gasified sufficiently tends to form fine slag in large particle size, while coal gasified insufficiently is more likely to form slag in middle particle size. To some degree, all these findings can supply a certain basis to the study of gasification process. Meanwhile, the medium-sized fine slag with the most content in fine slag has low gasification degree, large content of carbon, and large specific surface area and porosity, which still has a certain potential utilization value for the treatment and disposal of the fine slag.