Abstract: Using solar wind data with time resolution 64 s and SYM-H data with time resolution 1 min and considering the delayed geoeffectiveness of solar wind observed at L1, we reinvestigate the Interplanetary (IP) sources for the main phase of the geomagnetic storm occurred during October 18--19, 1998. The results indicate that only the sheath made contribution to the main phase of the geomagnetic storm. The Interplanetary Coronal Mass Ejection (ICME) started at 03:51 UT, October 19, 1998 and ended at 14:17 UT, October 19, 1998 having the southward interplanetary magnetic field with averaged value 16.8 nT made no contribution to the main phase of the geomagnetic storm. The proton density and the dynamic pressure of the ICME was only 5.2 cm -3 and 1.43 nPa respectively. The results indicate that the product of solar wind density and the south component of IMF determine the development of the main phase of a geomagnetic storm.