Most of the melon(Cucumis melo L.) breeding lines in Korea show andromonoecious (male-perfect flowers) sex expression, which requires laborious hand emasculation to produce the F1 seeds. There is a high demand for developing monoecious (male-female flowers) elite germplasm. The present study was carried out to develop molecular markers for selecting monoecious plants based on the CmACS-7 gene [a locus with 1-aminocyclopropane-1-carboxylic acid synthase(ACS) activity] responsible for ethylene synthesis and sex determination in melon. The full length sequences of the CmACS-7 were cloned from a monoecious inbred ‘Mo23’ and an andromonoecious inbred ‘Am24’. Sequence alignment revealed a major SNP(C170T) in exon1 and 18bp indel in intron4 of the CmACS-7, and a CAPS (SNP-C170T) and SCAR (ID4-18) were developed from the SNP and indel, respectively. A total of 453 F2 plants derived from ‘Mo23’ x ‘Am24’ were determined for their sex expression and genotyped using the SCAR marker. A Mendelian ratio of 3(monoecy): 1(andromonoecy) was observed from the F2 population, and sex type of 449 plants (except for four plants that showed incomplete monoecy) cosegregated with the SCAR marker, demonstrating that CmACS-7 is a single dominant gene conferring monoecy of ‘Mo23’. Allele variation of the CmACS-7 was evaluated by genotyping 114 melon accessions with diverse geographical origins using the CAPS and SCAR. C170T-SNP in exon1 of the CmACS-7 was highly conserved in melon germplasm and perfectly matched with the phenotype, whereas the 18bp-indel mutation in intron4 existed in various forms. The results demonstrated that CAPS marker SNP-C170T can be useful for marker-assisted selection(MAS) of monoecious melon plants