We statistically study 105 geomagnetic storms with a Dst peak value ≤ -50 nT during 1998--2001 to examine the influence of the interplanetary parameters -$overline{VB_{z}}$ and its duration Δt on the intensity of geomagnetic storms. About 33% of the events are associated with intense storms with Dstmin ≤ -100 nT. It is found that -$overline{VB_{z}}$ is much more important than Δt for the formation of geomagnetic storms. A stronger -$overline{VB_{z}}$ can produce a more intense storm, whereas a longer Δt can not. A simple empirical formula relating the Dst peak value to -$overline{VB_{z}}$ and Δt is obtained, which shows a good correlation (CC = 0.9528) between the estimate value and the observations. This formula suggests that a compressed Bs field tends to have a more prominent geoeffectiveness. Moreover, we also identify 33 large -$overline{VB_{z}}$ intervals with -$overline{VB_{z}}$ > 5 mV/m and Δt > 3 hours in the same study interval, and find that they all caused intense storms (Dstmin ≤ -100 nT) and 8/9 of the great storms (Dstmin ≤ -200 nT) were due to interplanetary compressed structures.