Abstarct: One way to measure the fluids flow inside the tubes is measuring by pressure difference that due to simplicity, low cast and being baxsed on proven scientific principles, it has many applications in measuring of gases and fluids flows. This measurement basis is that by positioning an element in fluid flow path, a pressure difference would be created on both side of this element, this easily measurable pressure difference in sides of element is proportional to the fluid flow rate. Flow meters that measure flow rate using pressure differences are including an orifice flow meter, ventury meter and cone flow meter. Determining of actual discharge of flow is a common goal in all these three flow meters. Discharge Coefficient (Cd) is used to transform theory discharge to actual discharge. In this thesis, the relationship between discharge coefficient and Reynolds number in above-mentioned flow meters were investigated. Computational fluid dynamics software of Ansys Fluent and ICEM-CFD software were used to analyze and produce of net respectively. This investigation was done in laminar and turbulent regime. For Reynolds numbers of higher than 20000, water and for Reynolds numbers of less than 20000, oil were used as fluid. Considering models for each flow meter, each model was run according to chosen Reynolds number and eventually discharge coefficient was obtained by determining pressure differences of between surfaces. To represent the results, discharge coefficient and Reynolds number were plotted on semi log graph in x-axis. Also in order to validate the results, the obtained results were compared with results of previous works. According to the results, discharge coefficient in ventury flow meter and crone flow meter had a specified trend proportional to Reynolds number but in orifice flow meter this trend was different.