This chapter is based on measurement of fat in adulterated milk by optical method, where we choose a binary PC structure with a symmetrically introduced defect. In this structure, the defect layer contains milk of different fat concentrations. We determine the transmission characteristics of such a PC and obtain the defect mode in the PBG region. By changing the fat concentration in milk, it is noted that the defect mode frequency increases linearly showing a prominent role, for an optical sensor for measuring the fat concentration of milk, as the fat concentration of milk sample causes change in refractive index. Such change in refractive index enables shifting in defect mode wavelength along with FWHM and QF. Here, the resonant wavelength of defect mode is considered as indexed parameter for measuring the fat concentration and check the adulteration in milk. Also, the sensitivity or performance of this optical sensor device is high, that is, 233.63 nm/RIU.