This research was carried out to investigate the effect of different levels of potassium (in nutrient solution), harvest season and plant density on yield and quality of strawberry cv. Selvain hydroponic system conditions. The tests were done in the greenhouse, and laboratories of horticultural science department of the university colleg of agriculture and natural Resources of Tehran University. Throghout the research, there were evaluated the effects of four levels of potassium at concentrations of 1.5, 2.6, 3 and 4.5 meq/L. The control consisted of half strength Quick nutrient solution (2.6 meq/L potassium). Three plant densities (12, 16 and 20 plants/m²) and two harvest seasons (winter and autumn) were considered in a completely randomized block design of three replications as regarded yield and fruit quality of strawberry cv 'Selva'. The main difference among solutions was observed in their potassium concentrations. Results revealed that, increasing potassium level in nutrient solution up to 3 meq/L caused an increase in TSS, TA, Vitamin C content and fruit potassium content, i.e. level of fruit potassium increased with increase nutrient in solution of potassium from 2.6 to 4.5meq/L. Planting densities of 12, 16 and 20 plants/m² did not affect yield and fruit quality. The effect of harvest season (autumn as compared with winter) was more pronounced on some fruit characteristics such as TSS/TA. However yield, TSS and TA were low in autumn harvest. According to the obtained results, modified Quick nutrient solution with 3meq/L of potassium became known as the most proper solution, but different plant densities did not exhibit any significant differences.