Continuous preparation of Mn-doped ZnS nanocrystals, which exhibit photoluminescence properties, by the hot-soap method was conducted in a thin tube flow reactor. The hot-soap method is a reactive crystallization from organometallic reagents dissolved in hot surfactant (tri-n-octylphosphine oxide : TOPO). While rapid mixing is generally essential for reactive crystallization, in the case of laminar flow in a thin tube reactor, diffusion-based slow mixing is expected to be dominant in the reaction field. In this study, the effect of introducing gas-liquid slug flow into the thin tube reactor was examined, with the expectation that the mixing performance would be improved by the internal circulating in the liquid slug. As the result, the reaction period (residence time) was found to be reduced compared to that without slug flow.