The utilization of steel slag mixed soil as a soil material for seaweed bed has attracted attention in recent years. In this study, a laboratory growth experiment of Zostera japonica (Z. japonica) was conducted to determine the effect of particle size of steel slag on the growth of Z. japonica. The results indicated that the growth of Z. japonica in the systems containing more than 2 mm of steel slag were similar to that of the control system without steel slag. Especially, in the system containing 2 to 4mm of steel slag both Z. japonica and diatoms on the surface of sediment grew well. However, in the system containing less than 2 mm of steel slag, the growth of Z. japonica was limited. It was considered that the increase of sediment strength and the lack of phosphorus due to fine particle slag addition caused the growth inhibition of root of Z. japonica. Therefore, when steel slag is used for seaweed bed, not only mixing ratio but also particle size of steel slag should be considered carefully.