A new high-sensitivity homodyne in-phase and quadrature (I/Q) -interferometer scheme for measuring the tilt change of a target is presented. The new tilt sensor is a Mach-Zehnder interferometer folded by the target, in which the phase change is induced by the in-plane tilt change of the target but is not sensitive to any other motions. The interferometer is specially designed to minimize interferences caused by environmental perturbations. The induced phase is directly measured by using the I/Q-demodulation scheme. The tilt sensor exhibits an excellent sensitivity 10 prad/Hz1/2 at a frequency slightly above 1 Hz and a 0.4 prad/Hz1/2 at a frequency higher than 30 Hz.