We report a low-loss, low-cost high-sensitivity all-fiber strain and temperature sensor based on mode interference in graded-index multimode fibers. Blueshifts with strain and temperature sensitivities of 18.6 pm/microstrain and 58.5 pm/°C have been observed. Experimental results show that smaller core diameter graded-index fibers display greater strain-induced peak wavelength shifts than larger core diameter fibers