Improvement of the optical fiber oxygen sensor based on the fluorescence quenching by oxygen gas was studied. Using the newly synthesized poly-l-menthylmethacrylate (PMtMA) and its blend of the PMtMA/PMP as the cladding layer, in which sensing dye was doped, the plastic optical fiber (POF) O-2 senser with high sensitivity and a fast response time could be realized. In addition this sensor didn't receive any affect by humidity.