A high speed phase modulator based on ferroelectric liquid crystal (FLC) is demonstrated. For uniform alignment and pure phase modulation, we propose a new FLC device using a short helical pitch material in a homeotropic alignment. This device is driven by periodic in-plane electrode stripes implemented on the surface of both cell substrates. As a result, we have obtained a large phase modulation (about 2.3 pi at lambda = 1.55 mu m under E = 2.5 V/mu m) and fast response time (< 200 mu sec). Together with the performance characterization of our device, we discuss the alignment breaking effect which occurs near the edges of operating region.