Abstract: A thunderstorm process in Beijing on 20 September 2008 is simulated using a 3-D charging-dischargingcloud model. The effects of big particles consisting of graupels, ices, hails and raindrops on lightning initiation are investigated.Temporal and spatial analysis on the model results, including the mass concentration and the charging velocity, shows that graupels and ices are the most important particles that affect theinitiation of most lightning.Because lightning always initiate in the region with mass distribution of graupels and ices.The charging velocities of graupels and ices also reach large values there.From the analysisof time series, those large charging velocities of graupels and ices appear when lightning initiate intensively.Hails may also be an important factor effecting the lightning initiation except for the early stage oflightning activity.The region of lightning initiation correlates partially with the mass contribution of hails.The period when the charging velocities of hails reach their large values chimes with the lightning activity.But hails meet their large values below the height of lightning initiation.At the early stage of lightning activity, the mass concentration of hails is very small.Raindrops locates beneath the region of lightning initiation from beginning to end.Their charging velocities become prominent after the end of lightning activity.So it is impossible for raindrops to affect the lightning initiation directly.In many cases, it can be the signal of lightning warning that some kind of strong echo reaches athreshold height.Through the effect of particles on lightning initiation above, it can be concluded that thestrong echo should be caused by graupels or hails.When graupels (hails) are brought to the upper level above the threshold height by updraft, graupels (hails) mixed with ices adequately and the strong chargingprocess occurs among them.The first lightning will initiate soon after that.