Data of the crustal thickness and Poisson's ratio from 58 broadband seismic stations in the North China Craton, measured by Xu and Zheng (2005, Chinese Journal of Geophysics, 48, 1077~1084) using the techniques of teleseismic receiver functions, have been carefully examined. Two types of correlation between the crustal thickness (H) and Poisson's ratio (ν) have been observed: with decreasing H, ν increases gently and linearly in the BaodingDatong and GuantingZhangjiakouZhangbei regions, while increasing abruptly and nonlinearly in the rest of the block (e.g., Northeastern Hebei province and the southern Taihangshan area near Shijiazhuang). The linear correlation is interpreted as due to the summed, opposing contribution of tectonic thinning of felsic crust and the addition of mafic rocks crystallized from underplated magmas to the bulk crustal ν. The abrupt increase of ν with decreasing H suggests that much larger thinning strains have taken place in the felsic upper and middle crust than in the mafic lower crust during MesozoicCenozoic tectonic extension. It is further inferred that basaltic underplating has been localized mainly in the Zhangjiakou and adjacent regions.