Context : We present a new primary transit observation of the hot-jupiter HD 189733b , obtained at 3.6 \mu m with the Infrared Array Camera ( IRAC ) onboard the Spitzer Space Telescope . Previous measurements at 3.6 microns suffered from strong systematics and conclusions could hardly be obtained with confidence on the water detection by comparison of the 3.6 and 5.8 microns observations . Aims : We aim at constraining the atmospheric structure and composition of the planet and improving over previously derived parameters . Methods : We use a high- S / N Spitzer photometric transit light curve to improve the precision of the near infrared radius of the planet at 3.6 \mu m. The observation has been performed using high-cadence time series integrated in the subarray mode . We are able to derive accurate system parameters , including planet-to-star radius ratio , impact parameter , scale of the system , and central time of the transit from the fits of the transit light curve . We compare the results with transmission spectroscopic models and with results from previous observations at the same wavelength . Results : We obtained the following system parameters of R _ { p } / R _ { \star } = 0.15566 ^ { +0.00011 } _ { -0.00024 } , b = 0.661 ^ { +0.0053 } _ { -0.0050 } , and a / R _ { \star } = 8.925 ^ { +0.0490 } _ { -0.0523 } at 3.6 \mu m. These measurements are three times more accurate than previous studies at this wavelength because they benefit from greater observational efficiency and less statistic and systematic errors . Nonetheless , we find that the radius ratio has to be corrected for stellar activity and present a method to do so using ground-based long-duration photometric follow-up in the V-band . The resulting planet-to-star radius ratio corrected for the stellar variability is in agreement with the previous measurement obtained in the same bandpass ( Désert et al . 2009 ) . We also discuss that water vapour could not be evidenced by comparison of the planetary radius measured at 3.6 and 5.8 \mu m , because the radius measured at 3.6 \mu m is affected by absorption by other species , possibly Rayleigh scattering by haze . Conclusions :