Terahertz (THz) technology has been paid intensive attentions in biological testing due to unique properties of THz radiation. Utilizing the enhancement effect of a metamaterial chip to a localized THz field, the sensitivity of current THz systems to biological tissues could be greatly improved. In this work, a metamaterial chip with a rectangular aperture array was introduced into a THz focal-plane imaging system to achieve the measurement of biological samples. The function of the metamaterial chip was theoretically simulated and experimentally characterized. By measuring two types of plant leaves, the imaging results with and without the metamaterial chip were compared and analyzed. The enhancement effect of the chip to the imaging contrast was verified. The technique was expected to further broaden applications of THz imaging in biological sensing.