Computational Fluid Dynamics (CFD) offers numerical modeling investigating the performance that can be used other than or in tandem with experimental investigation. In order to offer meaningful results, the grid sensitivity test or Grid Convergence Index are usually carried out to ensure the solutions converging. This paper presents both grid sensitivity test results and the GCI calculation of a same numerical model exercised to investigate the performance of a tidal turbine, the Hydro-Spinna, with different P/D ratio. The CGI calculation presented the fine grid relative error to be 2.34% for the power coefficient and 2.28% for the thrust coefficient at their optimal TSR respectively. It was found that turbine with lowest P/D ratio has the highest power and thrust coefficient as well as TSR operational range. The turbine with P/D = 0.43 gives a power coefficient of 0.32 at the optimal TSR of 2.25.