In this study, runner systems which have equal cross-sectional area and different forms of cross-sectional diffuser and non-diffuser runner system's effects on mechanical properties of A356 aluminium alloy, depending on the movement of the liquid metal, were investigated. The aim of the study is to reduce casting defects due to inclusion caused by runner systems such as gas or shrinkage porosity. Therefore, the horizontal runners have been extended by using diffusers having cross-sectional area equivalent with a different cross-sectional shape (square, rectangular and circular outlet). Tensile test specimens were prepared from the plates of casting A356 alloy and tensile tests were performed. Obtained results were examined Weibull statistical analysis. As a result, one hand, it is observed that the turbulence effect on liquid metal has been decreased the highest tensile results and Weibull modulus has been appeared on the samples which have horizontal runners produced by using a square section diffuser in designed horizontal diffuser. On the other hand, according to Scanning Electron Microscope (SEM) analysis on fracture surfaces, fracture has been observed in the area of intermetallic secondary phases (oxides etc.) and in area with gas or shrinkage porosity.