Polyaniline nanoparticles (PANI NPs) with size 45 nm were prepared based on chemical oxidation polymerization of aniline in acidic aqueous media by using ammoniumpersulfates an oxidant. Based on FTIR and XRD analysis for PANI NPs the structure was confirmed. The XRD analysis for PANI NPs shows the 100 and 110 crystallographic planes of the partially crystalline PANI NPs. PANI NPs color changeability property with acid/base forms exploited for optical pH sensor construction. At PANI optimum conditions, the pH sensor demonstrated a wide linear pH response range between pH 413 in 50 mM PBS. The pH sensor shows two linear calibration curves; at 870 nm and 425 nm where the optimum sensitivity of the pH sensor of 0.055 and the correlation coefficient (R2) of 0.98 (n=3) and %RSD=7.1 were achieved at 870 nm wavelength.