AbstractAchieving maximum repeater spacing in optical fiber transmission systems is compromised by the input power limit due to stimulated Brillouin scattering (SBS). In digital transmission, SBS causes bit‐error rate degradation. The calculation method for the SBS equation under steady state in a single‐mode optical fiber has been established here; boundary conditions and actual environment are both considered. A new SBS threshold has been proposed as the input optical power at which the backscattered SBS power equals the Rayleigh backscattering power at the fiber input end. Based on a detailed comparison with experimental results, it has been confirmed that the proposed threshold expresses accurately the bit‐error rate degradation due to the SBS; it is approximately 2.5 dB lower than the conventional threshold. Use of the proposed threshold makes accurate system design possible.