This study entitled "Determining Timing of planting watermelon (Citrullus vulgaris) Based on Water Balance In District Mendoyo area Jembrana" aimed at 1) Calculated water balance in the region Mendoyo Region 2) Calculate the crop water requirements, and 3) determine the appropriate planting time. This study uses statistical methods to calculate the average monthly rainfall and methods Thornwaite and Mather for water balance analysis. Based on Penman-Monteith method, the value of ETo area Mendoyo of the year amounted to 1,563 mm with the actual evapotranspiration (ETA) of 1495 mm. January to May is the period of surplus and deficit period is July and August. periods of surplus water in the area Mendoyo lasts for 6 months ie December to May. While the water deficit occurs during the period of three months, between July and September. Water re-charging period after a water deficit (recharge) at the start of October. Most water surplus value occurs in January amounted to 141 mm, whereas most water deficit is currently the most critical groundwater conditions occur in August by 25 mm. Planting of watermelon can be cultivated in December, where in December there is a surplus of water. And when the harvest is expected to do in February. Because in general, watermelon crop ready for harvest in the range of 80-90 days after planting the seed.