Sensitivity of microbolometer Terahertz (THz) focal plane array (FPA) is improved by one order of magnitude at 0.5–0.6 THz, comparing with the previous THz-FPA. This is achieved by modifying an optical cavity structure in a pixel, such that the optical cavity length is extended by a factor of 4–5 than that of the previous structure. Minimum detectable powers (MDPs) per pixel are measured at 4.3, 2.5, 0.6 and 0.5 THz for the THz-FPAs (640×480, 320×240) with the modified structure and those (320×240) with the previous structure.