The study of Determinent Factors for the Concentration of Petroleum Product Retails in Thailand was based on information during 1988-1992. Two indicators, Concentration Ratio and Herfindahl Index, were used to measure the level of Concentration of petroleum retail market. In addition, many factors included market share,number of service station,advertisement,assets and government policies were determined to study the cause-effect of the Concentration among four major companies, PTT, Shell, Esso and Caltex. From the study, Concentration ratio of the four major companies of gasoline and premium gasoline (plus ulg) trended to decline continuously. This was also applicable for high speed-diesel but the lower rate. The factors that impacted concentration ratio are classified into two categories, company factors and government policies. Company factors include service stations, price and advertisement with strategies of lower selling price and more number of service station, the big companies are able to occupy more market share. However, advertisement seemed to be reduced and this effect market portion of petroleum retailing business. According to the study,government policy was a significant impact of Concentration. After liberlizing petroleum business, the concentration ratio was reduced caused by more new entrances. From the study of growth rate of the business, premium gasoline (plus ulg) have the highest rate among three products. Secondly and lastly were hi-speed diesel and regular gasoline. In terms of individual firm, PTT had the best trend, Shell was the leader in premium gasoline (plus ulg). Esso lost its position in regular gasoline and hi-speed diesel. The figures show that PTT became the first in hi-speed diesel and regular gasoline sale.