The easier and more economical control of root-zone temperature (RZT) as compared to that of other environmental factors such as air temperature could be an effective solution to temperature stress for the crop plants in hydroponics. The present study was designed to investigate the effect of root-zone temperature on the growth and yield of cucumber (Cucumis sativus L.) plants in recirculating hydroponic system under greenhouse of dimension, 9m x 30m during three cropping periods of the year in Oman viz. summer (June-August), fall (September-November) and spring (February-May) during year 2016/2017 at Directorate General Of Agriculture and Livestock Research of the Ministry of Agriculture & Fisheries located at Rumais Barka of Oman. The plants were grown in perlite medium at root-zone cooled temperatures of 22ᵒC, 25ᵒC, 28ᵒC treatments besides the control i.e. root-zone uncooled temperature of 33ᵒC as control treatment. The treatments were arranged in Complete Randomized Design (CRD) with four replications. The results indicated that the crop at root-zone temperatures of 22ᵒC and 25ᵒC gave the superior performance in terms of plant height, leaf number, chlorophyll content, leaf area cm2, fruit number/m2, yield in ton per greenhouse (t/gh), fresh and dry weight of shoot and root with significant differences between the treatments in all three periods. Fruit yield varied from 4.5t/gh to 6.4t/gh for root-zone temperature (RZT) and from 4.2 t/gh to 6.8 t/gh for the cropping periods. The higher yields of 6.4 t/gh and 6.4t/gh were found under RZT of 22ᵒC and 25ᵒC, respectively and were significantly higher (p<0.05) than that under control (33ᵒC; 4.5t/gh). In respect of cropping periods, the crop during fall period (February-May) gave higher fruit yield (6.8t/gh) than that during summer period (4.2t/gh).  Thus the results indicated that cooling of root-zone temperature through nutrient solution is essential during high temperatures of summer (June-August) in Oman.