The objectives of the study were to estimate costs and margins, seasonal price variation and to test market integration of onion. Primary and secondary data were used for this study. The higher marketing cost was incurred by Beparis and the lowest by Aratdars. On the other hand, retailers earned the highest net marketing margins. Farmer's net share of onion in lean period was higher than that in peak period. Seasonal price variation of onion was the highest in Rajshahi and the lowest in Dhaka market. Price was the lowest in March-April and the highest in November. The range and coefficient of variations of seasonal indices in different periods imply that seasonal price variation of onion in Bangladesh decreased in the recent past. Analysis of market integration shows that onion market in Bangladesh was well integrated.