Precise theoretical results of Maker fringes in biaxial crystals are presented which include such necessary corrections as absorption, finite beamwidth effects, and multiple reflections. Their applications to the new nonlinear optical material, organometallic complex crystal thiosemicarbazide cadmium chloride monohydrate (TSCCC, point groupCs‐m), and the corresponding experimental values of its six independent nonlinear optical coefficients are given.