A semiconductor fiber-optic ring laser gyroscope (S-FOG) consists of a semiconductor optical amplifier (SOA) and optical fiber to form a ring cavity. The fiber ring cavity enables larger sizes and smaller scattering, while the SOA gain is shared by the clockwise (CW) and counterclockwise (CCW) propagating modes. When the S-FOG is rotated, a new beat signal called the Sagnac beat frequency is observed. We investigated the effect of the fiber ring cavity's length on detection characteristics. Detection sensitivity was not dependent on the number of laps of fiber ring cavity. However, when the cavity length became longer, the linewidth of Sagnac beat became narrower, and then accuracy of angular velocity detection improved. The relation between the linewidth Av of the Sagnac beat and cavity length P was proved to be Δ≈ -1.86.