We present a double cladding fiber interferometer consisting of a long-period fiber grating pair for simultaneous measurement of temperature and strain. The insensitivity of the inner cladding mode to the external contact enabled easy handling of the interferometer without affecting the interference; thereby, strain could be applied only to the grating-free region between the two gratings. We show that the relative phase of the interference fringes is solely dependent on the strain, whereas the ambient temperature causes a shift in the envelope of the fringes. The temperature and strain were measured with sensitivities of approximately 0.038 nm/° C and 1.05 rad/mε, respectively, without appreciable cross-sensitivity. © 2008 IEEE.