A four-core optical fiber is introduced as a strain based temperature sensor to investigate the phase shift based on the temperature variations. An interferometric fringe pattern is obtained by the coherent waveguides from the four cores. A small piece of a four-core fiber is winded around a solid stainless steel cylinder to form a tight circular loop, which is exposed to a temperature change from 50 °C to 92 °C. Shear strain due to the expansion of the steel rod at this temperature interval causes an optical path length difference between the inner and outer core pairs, resulting a total phase shift of 20.4±0.29 rad, which is monitored with a CMOS camera. Using the phase changes, two dimensional shear strain is determined.