An improved open-ended coaxialprobe technique for measuring electromagnetic prop-erties of coating materials with curved surfaces (bothconvex and concave) is studied. FDTD modeling indi-cates that the reflection coefficient of the loaded probeis more sensitive to concave surface samples than tothose with convex surface. In order to satisfy concavesurface materials testing, we proposed a technique toimprove the measurement accuracy by modifying thestandard coaxial probe. A ring patch is added at theend of extended inner conductor throughout the airgap between the probe and the material under test.The performance of the proposed technique is exam-ined. The derived ε* and μ* of several microwave ab-sorbing materials coated on prototype boxes by usingthe new probe are relatively in agreement with thepublished data.