By data mining from 3DFEM simulation and Rough Set Theory (RST), it was performed that the extrusion process and die structures effect on the quality of AZ80 magnesium extrudate. The weights of the effect can be obtained. The results show that the effect of the billet temperature on the product quality is dominate, and its average weight is 0.27. The second important parameter is the ram speed and its average weight is 0.22. In addition, it was also found that the effect of the die characteristic parameters on the extrudate is insignificant.