In this work we present an optical Fiber Bragg Grating (FBG) strain sensor appropriate for uniaxial compression test of rocks. The rocks are a complex aggregate of materials and may exhibit in some cases mineral grains of size bigger than the conventional electrical strain gages used. Furthermore, commercial surface mountable optical FBG strain sensors are packaged in planar configuration, which is not appropriate for the cylindrical rock cores in uniaxial compression test. The optical FBG sensors were designed and manufactured manually fitted for cylindrical rock samples. A calibration process was carried out in order to stablish the mechanical performance of the FBG packaging; later, a compression test was conducted on hard rock sample and the strain was recorded by the traditional electrical strain gage (ESG) and FBG system. Under this comparison, the FBG sensors reaches a larger recording area, sensitivity under 100 microstrain and a reliable strain transfer, all of which is appropriate to measure strain for heterogeneous rock composition and minerals grain size.