Modern agricultural technology such as greenhouses is suggested as one of the most effective solutions to save water. Over 40 hectares of land is cultivated under greenhouse in Oman. The agricultural sector uses more than 93% of the groundwater' in Oman. Many farmers use conventional ways of irrigation scheduling inside greenhouses. The main objective of this study is to determine the effective methods to estimate cucumber water requirements inside two types of greenhouses. Microclimatological data inside both greenhouses were measured using Delta 2e data logger. The ambient weather parameters were also measured using meteorological weather stations installed outside each greenhouse. Class A pan was also installed both inside and outside each greenhouse to estimate the daily evaporation. Four lysimeters, cultivated with single plant each, were installed inside each greenhouse. Drainage from each lysimeter was measured on daily basis to estimate the daily crop water requirement. Data obtained showed that Seawater greenhouse (SWGH) reduced the ambient temperature by 4.8°C while the conventional greenhouse (CGH) reduced it by 7.4°C. The Relative Humidity (RH) inside both SWGH and CGH were 23% and 20% more than ambient, respectively. CGH RH was always less than 60%, unlike SWGH, which indicated that SWGH is more suitable for cucumber growth since cucumber grow better at 60- 85% RH. Solar Radiation (RS) inside both SWGH and CGH were 4.7 and 4.3 kw/m2, respectively, which indicated that more shade is required in both greenhouses for a better cucumber growth. Calculation of Evapotranspiration (ET.) inside both CGH and SWGH using Penman-Monteith (P-M) equation gave daily ET, of 5.5 and 4.3 mm, respectively. The pan factor (Kpan) value inside and outside CGH was adjusted locally based on P-M equation and pan evaporation to obtain Kpan value of 0.82 and 0.9 respectively. Adjustment of SWGH outside Kpan gave value of 0.78. Based on P-M and Kc values, the ETc inside CGH was 4.1 mm/day while it was 7.9 mm/day outside the CGH during gravest stage of the crop. On other hand, the ETc values inside and outside SWGH were 1.5 mm/day and 4.1 mm/day, respectively, during flowering stage of the crop.