In this study, we used a micro-ultrasound (microUS) system that we developed in-house as an alternative method for tumor growth calipers. In addition, microUS was combined with small-animal positron-emission tomography (microPET) for tumor metastatic assessment. MicroUS provides anatomical information that can be used for tumor volume measurements while microPET is a functional imaging method with positron-emitting radiophamaceuticals, such as 18F-labeled deoxyglucose, [18F]FDG. In this study, microUS and microPET were performed in a mouse tumor longitudinal study (2-8 weeks), both with 3D tumor segmentation and volume measurements. Compared with vernier calipers, microPET generally overestimated tumor volumes during weeks 2-4 due to its inadequate spatial resolution. During weeks 5-8, standard deviations of microPET results were large due to tumor hypoxia or necrosis. On the contrary, microUS tumor volume measurements were more reliable as they were less affected by these factors. Nonetheless, microUS is not able to provide functional information similar to that provided by microPET. Therefore, microUS and microPET are complementary to each other as microUS has superior spatial resolution and microPET provides functional information, such as hypoxia or necrosis in the progression of the tumor. With image registration and fusion, the combination can be a valuable tool for cancer research.