In this paper we report the detection and characterization of HD 189733b, the peculiarity of this exoplanet is that the flow of the target star is decreased significantly (~ 3%) during the transit. We determined the radius of the exoplanet 1.27 ± 0.03 RJ, the impact parameter 0.70 ± 0.02, and the inclination of the orbit 85.4 ± 0.1°. The transit of the extrasolar planet HD 189733b is already done using the larger telescope. In this study, we used during the observation a telescope of modest size.