The western flower thrips (WFT), Frankliniella occidentalis , and the onion thrips, Thrips tabaci (Thysanoptera: Thripidae) are the important pests of tomato, cucumber, and ornamental plants and can cause severe damage to greenhouse crops as well as outdoor cultivation. Since it is commonly reported that the presence of these two thrips in the greenhouses together, in the present study, the presence of these two pests together and their seasonal fluctuations in the greenhouse of tomato, cucumber, Alstroemeria, Rose and Gerbera were studied. For this purpose, the seasonal density changes of both trips were determined by weekly samplings in these greenhouses from November 2018 to May 2019. During 21 weeks of sampling, 1050 samples were transferred to the laboratory and 1525 adults of thrips were isolated from plant materials and identified. In general, except for the Gerbera greenhouse, both species of thrips were found in all the greenhouses and the population of WFT were found always higher than T . tabaci population, probably because of shorter developmental time of WET, the increase in day length, and the higher flowering rate of the these plants. In the tomato, cucumber, and Rose plants, WFT females were the most commonly occurring thrips, but in Alstroemeria, five weeks of 21 sapling weeks, WFT males were higher than females, which may be due to the fact that female moved towards the flowers and their population was decreased by insecticide spraying. The effect of temperature on the population growth parameters was calculated by life- table parameters of both thrips at 25 and 27 ° C. The results of the experiments showed that the least growth periods for both WFT and onion thrips were achieved at 27 ° C, which were 10.02 ± 0.08 and 10.62 ± 0.08 days, respectively. The intrinsic rates of population increase (rm) at 25 ° C for both species were lower at 27 ° C. There was a significant (P 0.05) difference between the rm at two temperatures for WFT. The interspecific and intraspecific competition of these two species that were investigated at 25 ° C in different initial densities.at the laboratorycondition. The results indicated that the onion thrips was affected by interspecific competition in at population densities and the number of larvae produced per female was reduced, but this value did not reach to zero. The WFT was more successful in the interspecific competition onion thrips. At the same density of females, WFT produced more offsprings. The present investigation can be used in the pest management programs of onion thrips and WFT in different crop species. Key Words : Fertillity-Life table, Intrinsic rate of natural increase, Greenhouse products, Peat management