The isothermal coarsening behavior of primary solid particles in A356 aluminum alloy semi-solid slurry produced by angular oscillation (AO) technique was investigated. The comparison between the calculation and experimental results shows good quantitative agreement with Lifshitz-Slyozov-Wagner theory. The results show that the variation in shape factor and solid fraction is not significant, the average particle size increases with increasing holding time at the expense of the particle density. Ostwald repining is most likely the predominant growth mechanism in the AO-treated semi-solid slurry during rheocasting. The differences of coarsening occurred in rheocasting and partial re-melting process were also discussed.