An electro-optic probe tip that is made from LiTaO crystal to make tangentially two-dimensional electric-field (E-field) vector measurements is presented. We combine a new electro-optic modulation technique and a conventional one to resolve the two E-field components. The new modulation effect on the optical probing beam is caused by rotation of the principal axis the electro-optic crystal, which is proportional to the E-field. Inasmuch as there is no free charge involved in the axis rotation, rotation modulation of the axis can be as fast as conventional modulation. The principles are carefully derived, and an experimental system constructed, to measure two-dimensional E-field vectors on a test pattern. The results are in good agreement with those obtained with commercial software for electromagnetic simulation. The sensitivities of two tangential E-field components are 76 (mV/cm)/ radicalHz and 0.8 (V/cm)/ radicalHz, respectively. The root-mean-square error of an E-field directional measurement is 1.5 degrees .