The　validity　of　the　Brooke　formula　in　estimating　the　lattice　misfit　based　on　dislocation　network　structure　in　Nibased　superalloys　is　investigated.　The　misfit　value　of　a　sample　subjected　to　prolonged　thermal　exposure　as　derived　from　the　popular　Brooke　formula　is　below　that　determined　by　X-ray　diffraction　(XRD)　method,　whereas　the　misfit　estimated　using　the　Brooke　formula　for　a　sample　subjected　to　creep　is　greater　than　that　determined　by　XRD　method.　These　discrepancies　imply　the　invalidity　of　the　popular　Brooke　formula　in　two　aspects:　that　a　dislocation　network　is　not　always　mature　to　fully　relax　the　lattice　misfit　leading　underestimate　the　misfit　and　that　using　the　full　magnitude　of　the　Burgers　vector　overestimates　the　misfit.　To　overcome　this　problem,　a　modified　Brook　formula　using　the　{100}　projection　of　the　edge　component　of　a　Burgers　vector　as　the　effective　magnitude　to　relieve　the　misfit　is　proposed.　The　misfit　derived　from　the　modified　Brook　formula　is　in　good　agreement　with　the　values　determined　by　XRD　method　for　a　number　of　Ni-based　superalloys　studied.