The objective of this paper is to clarify the effects of thickness and thermal stability of turbulent boundary layer on aerodynamic characteristics of a tall building. A squared section cylinder with chamfered corner or rounded corners was set in a thermally stratified wind tunnel and its aerodynamic forces are measured by a high accurate load cell in unstably and stably stratified flow From these experiments, several conclusions are shown as follows, 1) The results in stably stratified flows are similar to those in neutral flows, because the mechanical turbulence ih the boundary layer weakens stably stratified flows. 2) In the case of unstably stratified flows, although the separated shear layers approach the side surface, the values of C_D and C_<Lrms> increase.