This work investigated a new colorimetric sensor for pesticide based on L-cysteine-modified silver nanoparticles (L-cys-AgNPs). The colorimetric sensor was prepared by reducing silver nitrate solutions in the existence of Diospyros blancoi leaf infusion which were then mixed by L-cysteine solution. The L-cys-AgNPs were characterized by UV-Visible Spectrophotometer measuring optical absorbance. Pesticides, namely monosultap and cypermethrin, were detected by mixing them with silver nanoparticles. The fresh L-cys-AgNPs had brownish yellow color inasmuch as surface plasmon resonance absorbance centered at 422 nm. NaCl stimulated color change from brownish yellow to clear which was relied on pesticide concentration of 20, 50 and 100 ppm. In the presence of cypermethrin, L-cys-AgNPs color turned clear and the peak absorbance dropped to 0.17 from 1.15. However, the effect of monosultap toward L-cys-AgNPs was negligible. This sensor allows a direct quantitative check for the detection of cypermethrin.