Soil water plays an important role in muskmelon (Cucumis melo L.) production, especially inside greenhouse. The object of this study was to determine the soil water content criteria at which growth and yield production of muskmelon would be optimal based on soil water measured by Time Domain Reflectometry (TDR) inside a plastic greenhouse when using drip irrigation. Four soil water content lest thresholds (relative to the percentage of field water capacity) for starting the irrigation (45%-I45, 55%-I55, 65%-I65, 75%-I75) were compared inside plastic greenhouse experiment in 2009. Amount of applied irrigation water for four treatments varied from 120.1 to 178.6mm. The results showed that muskmelon plant development, fruit production and quality were significantly affected under different irrigation water amounts. Higher soil water content enhanced vegetative growth through increasing the plant height and stem diameter. Variation in soil water content not only had effects on fruit size, but also had effects on fruit yield. The highest fruit yield and irrigation water use efficiency (IWUE) were obtained from the treatment employing the greatest irrigation thresholds and quantity of irrigation (I75). When the thresholds of soil water content were below 65% field water capacity during the whole growth period, the mean fruit weight and yield decreased significantly. Irrigation water amount significantly affected the flesh thickness and fruit total soluble solids (TTS), soluble sugar (SS), vitamin C (Vc), soluble protein (SP) and free amino acid (FAA) content. In conclusion, treatment I75 was the optimum irrigation schedule for muskmelon grown inside plastic greenhouse.