We report on tests of combined positron emission tomography (PET) and fluorescence molecular tomography (FMT) imaging system for in vivo investigation on small animals. A nude mouse was inoculated with MD-MB-231 breast cancer cells which expressed red fluorescent protein (RFP). For FMT system, reflective illumination mode was adopted with full-angle data acquisition. [18F]-Fluorodeoxyglucose ([18F]-FDG) was used as radioactive tracer for PET. Both data were acquired simultaneously and then reconstructed separately before fusion. Fluorescent tomography results showed exactly where the tumor was located while PET results offered more metabolic information. Results confirmed feasibility for tumor detection and showed superiority to single modality imaging.