What causes corruption? Most literature on corruption take into account the role of culture and socio-economic development though few studies have not even begun to look for another explanation such as institutions in explaining this corruption menace. Within institutionalist literature, there are generally two strands which center upon two different concepts in explaining corruption-- accountability and personal vote. Yet, the theoretical framework in this study will rely exclusively on the concept of personal voting since it would be more compatible with the nature of research used in this study—within country analysis. By exploring Thailand’s three general elections for the lower house (1992, 1995, 2007) with the same type of electoral rules—block vote, the study aims to examine the impact of district magnitude (elements of electoral rules: ballot structure, district magnitude, electoral formulas) as a key determinant of the level of corruption. The effect of district magnitude on corruption will be analyzed against the backdrop of the block vote system controlling for year and region. Based upon the personal vote argument, the study contends that in the context of block vote, large district magnitude would put more pressure on a candidate to distinguish himself/ herself from his/her party’s teammates as well as candidates from different parties, which in turn provide the likelihood for the use of corruption as one of their electoral strategies. The findings from this study show that the levels of corruption at a nationwide level for all three years are relatively the same. More importantly, the study found that district magnitude is related with corruption, that is, a large district magnitude is associated with a greater degree of corruption. Finally, the study shows that region is strongly associated with the level of corruption—highest in the Northeast and lowest in the South and in Bangkok.