This article studies the heating variation of the sample placed on various relative positions of the turning tray in the microwave oven cavity. We find the distribution regularity of heating points that are of insignificant variation statistically, and determine Cu content in the sample GBW07233 and Cu and As contents in the sample NIES SRM No. 1 that are digested on these points. The determinations prove that the precision and accuracy of the determination results will not reduce, even though there are insignificant variations of temperature on these points.