A three-axis mosaic surface encoder, which can measure the X-directional position and the Y-, Z-directional straightness of the linear stage in a long range, is proposed. The three-axis mosaic surface encoder is composed of multiple scanning probes and a mosaic scale grating. The mosaic scale grating is constructed by placing the multiple two-dimensional reflective-type scale gratings in a line to obtain long measurement range along the in-plane direction of a scale grating. In addition, the optical configuration of the encoder head, which employs multiple scanning probes to be projected on the mosaic scale grating, is proposed so that a long range measurement can be achieved by the mosaic scale gratings. With the proposed encoder head, the measurement information of the X-directional displacement can always be obtained from the probe which is not on the gaps of the mosaic scale grating. The proposed configuration was applied to the three-axis surface encoder, and basic performances of the proposed three-axis mosaic surface encoder are experimentally investigated.