A new method is used to measure optical opacity of exhaust gas from diesel engine. Measurements of opacity performed through opacimeter are compared with measurement of laser extinction obtainable near turbocompressor group of diesel engine, under stationary speed and load conditions. For this purpose an optical based on laserfotodetector is used. Laser beam goes through an optical window located along the exhaust pipe, after the interaction with diesel particulate, is detected by means of a silicon photodetector which supplies a photocurrent proportional to laser intensity. This kind of measurement allows, using Lambert-Beer's law, to compare the opacity values obtained with traditional opacimeter and those obtained with laser beam extinction. Three wavelengths are investigated, 635nm 785nm and 1523 nm, and the opacity values are plotted as a function of opacity given directly by the opacimeter. The empirical correlations obtained are exponential equations, which exhibit the following form: N1=K N^h, where Ni and N are respectively laser beam opacity and opacity given by opacimeter. K and h are factors depending on wavelength, particulate, engine and load conditions. These measurements are obtained in real time, data are acquired with a sample rate of 25 kHz, rather than the opacimeter which had a response time of about 0, 1 seconds while the opacimeter had a response time of about 0, 1 s. This real time opacity measurement methodology could be used, as starting point, to develop an on-board diagnostic device able to measure the exhaust opacity of diesel engine. The on-board diagnostic device for diesel engine will be obligatory from 2005.