According to the 3-D hail-category hailstorm model,this paper studies the in-cloud microphysical structures and the hail size distribution characteristics of two multi-cell hail processes occurred in Xining,Qinghai on 20 July and 5 August 1979.Results show that:1)The mixing ratios of cloud water and rainwater present the characteristics of first increasing then decreasing with the development of hail cloud.The hail is mainly formed by frozen undercooling raindrops in the two cases and grows up by deposition and by collecting super cold cloud water.2)The width of hail size distribution is related to updraft.The slantwise updraft is beneficial to hail recirculation growth and leads to wider width of hail size distribution in the former.In the latter,there is not the slantwise updraft,which is not beneficial to hail recirculation growth and leads to narrower width of hail size distribution.