Fused Deposition Modeling (FDM) allows the creation of specific shapes by stacking materials, which makes it difficult to check the quality of the products. Using an X-ray CT scanner to inspect voids inside products that affect mechanical properties is an effective method. However, traditional void analysis method is difficult to extract voids from the CT volume. In this research, the toolpath, which contains the ideal shape of the product, is used to improve the void extraction. Besides, since the toolpath represents the FDM process, it can be used to infer the formation mechanism of each void. Based on the toolpath, a novel void classification is proposed that can help the user rapidly determine the cause of each void and find ways to avoid them. Products with different infill densities are inspected, and the results are shown by slices.