Surface Plasmon Resonance (SPR) sensors based on optical fibers have attracted a great attentions due to high sensitivity measuring of physical and biochemical parameters. In this paper has been investigated SPR optical fiber sensor containing nanocomposite that have Platinum nanoparticles. Numerically, we calculated the sensitivity of this sensor with nanocomposite layer containing platinum nanoparticles and with the platinum layer, separately. The results show that the sensitivity of sensor increases with increasing the thickness of nonocomposite layer and with increasing the platinum nanoparticle fraction volume. Also, we show that the sensitivity of sensor with nanocomposite layer containing platinum nanoparticles is more than the sensitivity of sensor with platinum layer for all value refractive indexes of sample media.