The number of screenhouses has increased in recent years in many areas of the world especially in the arid and semi-arid regions including the Arabian Peninsula countries like the Sultanate of Oman. That is due to the advantages of screenhouses over the greenhouses especially between October and April. This paper presents the results of an experimental work aimed to evaluate the effects of screenhouse microclimate on cucumber grown on hydroponics. Two types of substrate growing systems, Wood Straw (WS) and Date Palm Straw (DS) were evaluated. Yield in kilograms and number of fruits per plant were monitored. Cucumbers planted in WS gave significantly higher number of produced fruits per plant than those planted in DS system. The yield was also larger with WS but not significantly different from the DS system. The average inside temperature was warmer than outside by 0.4-3°C during January and February colder than outside by 0.2-0.8°C during March and April. The inside vapour pressure deficit (VPD) was always lower than outside by 0.2-1 kPa. The average transmissivity of the screenhouse was 0.53. Average temperature at the front section of the screenhouse was always close to that at the back sections. Relative humidity, on other hand, was always higher (max. by 5%) than on back section, which explains the lower values of VPD at front section when compared to the back sections during all the study. The better climate for cucumber in the front section produced a higher yield in the front of the screenhouse.