This study examined the synergistic effect of carbon fiber (CF) addition on the electromagnetic wave shielding characteristics of carbon nanotube (CNT)-cement composites. Effect of the amount and length of CF on the electrical conductivity, electromagnetic interference (EMI) shielding effectiveness (SE), and complex permittivity of the composites were systematically investigated. The electrical conductivity was determined by means of a digital multimeter, and a network analyzer was used to measure the EMI SE and complex permittivity of the composites in the frequency range of 0.1 to 18 GHz. The electrical conductivity and complex permittivity of the composites incorporating 3 mm CF or 6 mm CF were shown to be proportional to the CF content, while the effect of CF length on the EMI SE was more pronounced as the CF length increased from 0.1 mm (aspect ratio 14) to 3 mm (aspect ratio 417). Enlargement of the aspect ratio of CF led to the enhancement of electrical conductivity and absorption loss of the composites, attributing to a notable increase in the SE.