The difference between lycopene and phytohormone levels among diploid, triploid and tetraploid plants of two watermelon cultivars during fruit growth and ripening was studied. The expression pattern of five genes(phytoene synthase(PSY1), phytoene desaturase(PDS), ζ-carotene desaturase(ZDS), carotenoid isomerase(CRTISO), and lycopene β-cyclase(LCYB)) was analyzed in details. In red-fleshed cultivar Mimei, lycopene content increased rapidly from 25 to 35 days after pollination(DAP), and then decreased at 40 DAP. Triploid and tetraploid fruit had higher levels of lycopene than diploid. Moreover, triploid tended to contain more lycopene than tetraploid during fruit growth and ripening stages. However, little amount of lycopene(0–2 mg kg–1 fresh weight(FW)) in yellow-fleshed cultivar Huangmei was found during all fruit development stages. In Mimei, transcript level of PSY1 was generally higher than the other four genes, and LCYB gene expression was the lowest among all five genes being tested. PSY1, CRTISO and LCYB genes showed higher transcript levels in polyploid than in diploid fruit. By contrast, in Huangmei, transcript level of LCYB was not the lowest, but only lower than that of PSY1. PSY1, CRTISO and LCYB genes showed higher expression levels in diploid than in polyploid fruit. In Mimei, the negative correlation between gibberellane(GA) content and lycopene accumulation was determined in all three different ploidy fruits, while a positive correlation was observed between abscisic acid(ABA) content and lycopene accumulation only in diploid watermelon. These results indicated that different lycopene contents in different ploidy watermelons is regulated by the differential transcription expression of the lycopene metabolic genes and phytohormones. The difference between lycopene and phytohormone levels among diploid, triploid and tetraploid plants of two watermelon cultivars during fruit growth and ripening was studied. The expression pattern of five genes (phytoene synthase (PSY1), phytoene desaturase (PDS), ζ-carotene desaturase In red-fleshed cultivar Mimei, lycopene content increased rapidly from 25 to 35 days after pollination (DAP), and then decreased at 40 (ZDS), carotenoid isomerase (CRTISO), and lycopene β- cyclase DAP. Triploid and tetraploid fruit had higher levels of lycopene than diploid. Moreover, triploid tended to contain more lycopene than tetraploid during fruit growth and ripening stages. However, little amount of lycopene (0-2 mg kg -1 fresh weight (FW) ) in yellow-fleshed cultivar Huangmei was found during all fruit development stages. In Mimei, transcript level of PSY1 was generally higher than the other four genes, and LCYB gene expression was the highest among all five genes bein gated. PSY1, CRTISO and LCYB genes showed higher transcript levels in polyploid than in diploid fruit. By contrast, in Huangmei, transcript level of LCYB was not the lowest, but only lower than than that of PSY1. PSY1, CRTISO and LCYB genes showed higher expression levels in diploid than in polyploid fruit. In Mimei, the negative correlation between gbeberellane (GA) content and lycopene accumulation was determined in all three different ploidy fruits, while a positive correlation was observed between abscisic acid (ABA) content and lycopene accumulation These results indicate that different lycopene contents are in different ploidy watermelons is regulated by the differential transcription expression of the lycopene metabolic genes and phytohormones.