Plastic mold design is a process to determine the initial shape of a product made of plastic material. Flexyble cup seedling design products for ornamental plant nursery media are made using plastic material Low Denstiy Polyethylene (LDPE) which has mild and elastic properties. This design aims to produce a runner system type design that can work in a balanced manner in a Injection Mold simulation. Produces the fastest cooling system design to cool the cold runner so that it is faster in the production process than 3 types of cooling system designs. Produces core and baffle designs on a Three Mold Plate system that is easy for assembly and manufacturing. The software used to design is Autodesk Inventor 2016 and product simulation software using Moldflow Plastic Insight 2015. To design the mold requires several stages, namely identifying products, determining cooling layouts, and simulating the results of data into moldflow software. The second stage is to design the construction of the three mold plate mold system. The third step is to draw a design construct in a two-dimensional (2D) image. Analysis results on moldflow plastic insight, showed the results of comparisons on 3 types of cooling layouts are very significant. However, if seen from the results that have been obtained from the simulation, for parameters time to frezze parts, circuit metal temperature, circuit heat removal efficiency, time to freeze cold runner and deflection all effects, cooling layout 2 has better results than 2 types of variations another cooling layout. For cooling layout selection, type 2 cooling layout was chosen, because the cooling layout resulted in the simulation results of the Circuit metal temperature of 28.47 ºC, Circuit heat removal efficiecy of 0.95, Time to freeze cold runner of 36.97 seconds, and Defflection all effect amounting to 0.18 mm. Among the 3 cooling layouts, the most efficient and good cooling type 2 layout is chosen for the flexyble cup seedling mold.