White smoke emission is observed at tailpipe of diesel vehicle when unburned hydrocarbons (HCs) are adsorbed on diesel oxidation catalyst (DOC) under low exhaust gas temperature. The purpose of this study is to get better understandings of white smoke emission derived from HCs and improvement of emission level. Firstly, HCs component and particle size distribution of white smoke emission have been analyzed. It was clarified that semi-volatile organic compounds and water are condensed around soluble organic fraction with submicron size. Additionally, relationships among behavior of white smoke emission and the amount or quality of HCs adsorbed on DOC were investigated by change of zeolite content in DOC. It was found that the adsorbed ratio of heavy HCs having higher boiling point than that of n-hexadecane becomes larger with decrease in zeolite content under exhaust gas temperature of 120 degC, leading to dense white smoke emission at same amount of adsorbed HCs on DOC. In addition, it was assumed that the zeoliteless DOC has a higher catalyst activity compared to zeolite containing DOC when HCs adsorbed under the gas temperature of 180 degC, which results in drastic reduction in white smoke emission, although HCs and carbon monoxide (CO) conversions during New European Driving Cycle mode become worse compared to conventional DOC specifications. Based on these experimental results and considerations, mechanisms of white smoke generation dericed from HCs were assumed and confirmation testing was conducted using by alumina with small specific surface area (SSA). As a result, it was found that adopting small SSA alumina on DOC achieves the breakthrough of trade-off between white smoke emission and HCs and CO conversion performance.