Glass mat thermoplastics (GMT) have the superior property/price ratio, they can be used in many industrial aspects. In this paper, through a 100mm-deep mold, the glass mat distribution, fiber orientation and mechanical properties of the part were studied under different blank designs. In conclusion, (1) Blank design has effect on the mat distribution and fiber orientation, and finally on the mechanical properties of the part. To a part, there is an optimum blank design, which will not greatly decrease the mechanical properties of the sheets in the final part. (2) In compression molding, interlaminar slippage, blank ejection and resin solidification result in mat distribution and fiber orientation.