The reduced magneto-hydro-dynamical (MHD) equations show that the pressure
gradient together with the magnetic-field curvature as a force drives the p
lasma short-wavelength MHD instability in a toroidal system. This paper dis
cusses the role of magnetic curvature of tokamak plasmas in some detail. It
is found that the region of bad magnetic curvature is enlarged due to the
presence of poloidal magnetic curvature. Especially, there exists a strong
MHD instability in the upside or the downside of tokamak.