Background A growing number of investigations have explored the contribution of cause of death to socioeconomic inequalities in mortality in Europe and North America, but few such studies have been performed on Asian populations. Objectives To analyse the socioeconomic inequality of cause-specific death rates from both an absolute and relative perspective, and to evaluate the contribution of cause of death to total mortality inequality in South Korea. Methods Data were obtained from public servant health insurance beneficiary records. 1 403 297 subjects aged 35-64 years were followed for 9 years. Health insurance premium levels were used as a socioeconomic position indicator. The outcome variables were all-cause, 11 broad causes and 41 specific causes of death. Mortality differentials were examined using causespecific age-adjusted mortality, relative indices of inequality, and slope inequality indices. Results Graded inverse associations between income and mortality were found for most, but not all, specific causes of death. The major contributors to income differentials in total mortality in men were liver disease (15.4%), stroke (12.8%), land transport accidents (10.0%), lung cancer (7.1%) and liver cancer (7.0%). In women, stroke (30.7%), diabetes (9.1%), land transport accidents (6.6%), liver cancer (6.0%) and liver disease (5.1%) were important. Conclusions The contribution of the cause of death to socioeconomic inequality in mortality in South Korea differed from Western countries. To develop a policy to reduce the magnitude of socioeconomic inequality, an understanding of the major causes of death that contribute to mortality inequality is required.