A Volume of Fluid (VOF) method is used to study the breakup of droplets in T-junction geometries. Symmetric T-junctions, which are used to produce equal size droplets and have many applications in pharmacy and chemical industries, are considered. Two important factors namely "breakup time" and "breakup length" that can improve the performance of these systems have been introduced. In addition a novel system which consists of an asymmetric T-junction is proposed to produce unequal size droplets. The effects of the channel width ratio and the capillary number on the size and length of the generated droplets and also the time of the generation have been studied and discussed. For simulation the problem, a VOF method used and for verifying the accuracy of the simulation the results compared with two analytical researches and a good agreement was found. The results indicate for the systems that generate equal size droplets, in a specific Capillary number (in our case 0.02) the performance of the system is in its optimum condition. Also for the systems that generate unequal size droplets, in large capillary numbers a wider range of droplets with different sizes can be produced.