The influence of microstructure on the magnetic properties was experimentally investigated to estimate the creep damage in ferritic 11Cr-3.45W steel as an applicable research for the structural health monitoring. The microstructure showed significant changes in precipitate size (d_p), precipitate number (N), dislocation density (ρ) and martensite lath width (ω) during creep. The magnetic coercivity decreased with creep duration and depended strongly on precipitate number and lath width. However, the magnetic saturation and remanence did not change after 500 h creep duration. The correlation between the microstructural changes and coercivity was discussed.