A high sensitive Mach-Zehnder Interferometer (MZI) based fiber optic sensor for the measurement of sea water salinity has been demonstrated in this research. At first, femtosecond laser is used as the source and its performance in sensing salinity of sea water is analyzed. Secondly, a Carbon monoxide (CO) laser operating in the spectrum of 4.8 to 8.3 pm is used to excite the arms of MZI. It has been observed that wavelength shifts related to attenuation dips are highly sensitive to salinity of sea water when CO laser is used as the source. Using simple and robust schematic design, sensitivity of 0.01pm/ppt is achieved for salinity range of 31 to 37 ppt at a constant temperature of 27°C. The all-optical MZI salinity sensor indicated here is easy to fabricate, low cost, highly sensitive and finds application in the field of oceanography.