This study investigates how ideological extremism is portrayed in online and offline communications by using the case of South Korea. We evaluate whether any forms of ideological biases are observed among Twitter users in Korea. We also examine how users are ideologically distinguished from one another in terms of their online political behavior. While most of the existing literature has relied heavily on survey data, we address these questions by means of both survey data analysis and Twitter data analysis using 13,500 tweets during the 2012 Korean presidential election. We find that 1. Twitter users tend to hold more extreme attitudes compared to non-users and the general population in Korea；2. There are more progressive tweets than conservatives ones in Korea； 3. Korean Twitter users differ from each other in terms of their online behavior according to their partisan and ideological preferences.