This paper investigates how crop diversity or product mix relate to the productivity in Korean crop production. I hypothesize that the degree of flexibility in land allocation across different crops determines the productivity of crop production. By using the province-level data from 1960-2018 for 51 crops, I estimate the effect crop diversification, measured as the concentration index of planted acreages across crops, on the province-level productivity. To deal with potential endogeneity of the diversification index, I employ panel instrumental variables and fixed effectsmodel by exploiting temporal lags in the crop diversification index. The estimation results from the most preferred specification indicates that a) the crop diversification index has a positive and significant effect on the province-level productivity and b) the effect becomes significantly larger in the periods after 2009.