Temperature dependent refractive index of DNA-cetyltrimethylammonium chloride (CTMA) thin-solid-film was measured 20 to 90℃ to obtain its thermo-optic coefficient of -3.6×10<sup>-4</sup> (dn/dT). DNA- CTMA film has high thermosoptic coefficient than other polymers. The film was deposited on coreless silica fiber (CSF) to serve as a multimode interferometer optical fiber temperature sensor. It is immersed in a water that changed temperature from 40 to 90℃. It has sensitivity of 0.25nm/℃.