The fluidity of A356 aluminum alloy was experimentally determined at the melt temperatures and vacuum degrees by a series of suction fluidity tests. In order to achieve different cooling rates during the test, quartz tubes, as well as stainless steel tubes, were employed as the fluidity channels. As the melt temperature increased from 650 to 730 ∘C, fluidity lengths either linearly increased from 26 to 36 cm or parabolically increased from 13 to 29 cm when quartz tubes or stainless steel tubes were employed, respectively. As the vacuum degree of the fluidity test increased from 0.005 to 0.03 MPa, fluidity increased from 25 to 43 cm in quartz tubes while the smaller increase in fluidity from 20 to 31 cm was observed in stainless steel tubes. Shorter fluidity lengths in stainless steel tubes than those in quartz tubes under the same fluidity measurement condition were due to faster solidification speed confirmed by microstructural analysis. In order to predict the fluidity of the A356 alloy obtained from the suction fluidity tests, a mathematical model was developed based on heat and mass transfer equations coupled with thermodynamic calculations by ChemApp software. The simulation results show good agreement with the fluidity length obtained in the present study. From a series of model calculations, the effects of casting parameters on the fluidity of the A356 melt were discussed.