The stress field near a cusp is examined by Taylor series expansion and complex potential representation of stresses. The conformal mapping function m(zeta) satisfies the conditions m'(zeta0) = 0, m''(zeta0) not-equal 0 at a cusp zeta0. The result shows that a cusp behaves like a crack, i.e. it has R-1/2 singularity. Therefore the stress intensity factor concept can be used to characterize the stress field in the neighborhood of a cusp.