This paper analyzes the location choice of foreign manufacturing firms in 15 Korean regions. Using the data set on foreign invested enterprise in the period of 2006-2009, we explore the role of agglomeration economies, trade union strength, and their interaction in determining the regional location choice of FDI, after controlling for production cost, labor quality and accessibility, and other factors. Employing the nested logit model, we find that regions with higher agglomeration, higher human capital, and lower trade union strength tend to attract more foreign manufacturing investment. Furthermore, we explore the interplay of agglomeration economies and trade union strength. Interestingly, empirical analysis shows that positive effect of agglomeration economies are stronger in regions with stronger trade union, in the case of location choice of foreign knowledge-based manufacturing firms. This result suggests that agglomeration of firms belonging to the same industry could play a more significant role in regions with stronger trade union.