This paper presents a measurement method for straightness errors based on phase sensitive detection technique. This system consists of a single-mode fiber pigtailed laser diode, a corner cube, a position sensitive detector (PSD), and electronic components1. The beam from single mode fiber is stable, which can be used as a datum for straightness error measurement after collimation. The PSD is used to receive the beam reflected by the corner cube. To eliminate the influence of background light, the laser diode is modulated by a sinusoidal wave current and phase sensitive detection technique is developed to demodulate the signal. Compared with a laser interferometer, the measurement accuracy is better than +/-0.4 μm. A repeatability test is conducted to assess the system stability by measuring a stage three times, showing that the repeatability is better than 1 μm. This system can be easily assembled to measure straightness errors of a carriage in workshop. 1 μm. This system can be easily assembled to measure straightness errors of a carriage in workshop.