Abstract: This study ascertained mycelial compatibility groups (MCGs) and pathogenicity variations of Sclerotinia sclerotiorum isolates sampled in sunflower from China, Canada and England. For this purpose, 205 isolates (5 from England, 33 from Canada and 167 from China) were grouped in 39 MCGs. Out of 39 MCGs, 64% were represented each by a single isolate. Sclerotinia isolates from the three countries` were incompatible between each other. Differences among MCGs were observed by comparing their differences in radial growth, sclerotial yield on PDA and detached leaves, aggressiveness and production of oxalic acid and total acids. Significant differences were found in radial growth, sclerotial yield, aggressiveness, production of oxalic acid and total acids within and among MCGs (p < 0.001) regardless of their geographic origins. Correlation analysis indicated that S. sclerotiorum aggressiveness was positively related to oxalic acid production (r = 0.739, p <0.01) and negatively related to pH (r = -0.436, p <0.01), but not to the radial growth and the yields of sclerotia on PDA and detached sunflower leaves. Significantly, sclerotial yield on PDA was positively correlated to the sclerotia weight produced on detached sunflower leaf. There was a highly negative relationship between pH values (negatively indicated total acids secretion) and oxalic acid production (r = -0.669, p <0.01), suggesting that oxalic acid contributes most to the total production of acids released by S. sclerotiorum.