A major part of the flow field has no complicated behavior in many laminar & turbulent flows which can use large mesh and simplified governing equation for numerical simulation. In order to reduce memory and CPU time, the flow field was decomposed to several blocks with different size and dimension in each block. A two dimensional backward facing step was considered in this research that used three different type of blocking in numerical modeling. Computer memory and CPU time consumption in addition to numerical result of main fluid parameters. Consideration showed that, suitable combination of used blocks led to the results with the same accuracy as model was employed only one block and experimental results, in addition to the reduction of memory and CPU time consumption.