Reflection phase characteristics of cylindrically curved high impedance surfaces (HISs) are examined. Due to the non-periodicity of the problem, full wave solutions can be time consuming. To overcome this problem, an approximate semi-analytical method, which assumes a homogenized model for the curved HIS, is developed. The model parameters can be extracted from the reflection properties of the flat HIS. For the cases where only Floquet currents are excited, the reflection phase diagram of a curved HIS is independent of the curvature. However, the surface waves generated on HISs, due to their periodic geometry, distorts their reflection phase characteristics within specific frequency intervals. In those intervals, the reflection phase changes as a function of radius of curvature and size of the HIS. These effects are not observed for the flat cases because of the lower radiation resistance of the surface waves. In this paper, the normal incidence case is considered for TEz and TMz polarizations.