We examine the effect of preferential tariffs on external trade liberalization empirically in a 23 large group of nations. We use HS 6-digit industry-level data on applied most-favored nation(MFN) tariffs and averaged preferential tariffs(PRF) over 1988 to 2010. As a result, we find that preferential tariff reduction in a given sector leads to a reduction in the external(MFN) tariff in that sector. That is RTAs are likely to be building blocs to external trade liberalization in most of sample countries, in the sense that there is a complementarity effect between preferential and MFN tariffs.