Cleavage structure cuts across the members of a state and affects the electorate's choice of parties and candidates. Social cleavage structure can be stable or go through changes depending on on-going social change as well as political parties' electoral strategies. With the collapse of communism and the advent of the third wave of democracy, many countries in Eastern Europe, Latin America, and Asia have experienced rapid social change. New political parties have formed, and democratic elections have been introduced in most of these countries. With rapid social change and the resulting shifts in party strategies, do we also expect rapid change in social cleavage structure in these countries? Or does cleavage structure change only gradually and remain stable over a short-term period? In this paper, we attempt to answer these questions by examining the relative importance of three cleavages commonly claimed to have relevance to politics in the 2000s—regionalism, ideology, and generational differences—in voters' choices in Korea. We analyze the voting behavior of the electorate in the 16th and 17th National Assembly elections, in 2000 and 2004, respectively, by employing multinomial logistic analyses of voter survey data. Our analysis indicates that the relative importance of various cleavages changed during the early 2000s: the influence of regionalism remained strong, but its intensity declined; the importance of ideology grew; and a new cleavage of generational differences appeared. Our study shows that the cleavage structure is fluid even in a short-term period, especially in new democracies, as citizens and elites adjust their behaviors in response to changing social situations. Political parties may also introduce a particular electoral strategy to induce change in the cleavage structure.