A novel carbon nanofiber (CNF)-reinforced syntactic foam (CNFRSF) was fabricated. The electrical conductivity and electromagnetic interference (EMI) shielding effectiveness (SE) of CNFRSF with various volume fraction of CNFs (0-2.0 vol%) were evaluated experimentally. The measurement of EMI SE was carried out in a frequency range of 30 MHz to 1.2 GHz. The experimental results showed that the SE of CNFRSF increases with increase of CNFs content and CNF is more effective in providing EMI shielding compared to chopped carbon fiber and long carbon fiber due to the larger aspect ratio. The CNFRSF having 2.0 vol% CNFs has a SE of 25 dB, which is good enough for most practical applications. Compared to CNFs composites, the EMI SE of CNFRSF has been enhanced by the presence of hollow carbon microspheres. The underlying mechanism is discussed in detail.