The co-dominant InDel markers were successfully explored based on the insertion-deletion （InDel）mutation in the major gene/QTL conferring powdery mildew resistance in cucumber（Cucumis sativus L.）. The InDel marker InDel-MLO1 was co-segregated with the major gene for powdery mildew  resistance through linkage analysis. Known of powdery mildew resistance phenotype，24 cucumber lines originated from various geographical regions of the world were tested by the InDel marker. The results showed that the genotypes of InDel-MLO1 were consistent with their resistance phenotypes in 21 cucumber lines，which indicated that this causal InDel mutation for powdery mildew resistance occurred frequently during the evolution and domestication
of cucumber. Therefore，our finding confirmed that the InDel marker is very useful for identification of cucumber powdery mildew resistance and molecular marker-assisted breeding for most of the cucumber lines，and thus，the process of powdery mildew resistance breeding in cucumber will be accelerated.