46 inbred NIH mice were infected by chloroquine resistant strain of Plasmodium yoelii. The ultrastructure changes were observed under the administration of ketotifen(10mg· kg- 1· d- 1 ) and chloroquine( 10mg. kg-1 · d- 1 ) combination and one after another respectively. The effect of taking ketotifen and chloroquine combination showed that parasites died rapidly with a few of intermittent membranes and vacuoles. The effect of taking two kinds of drugs one after another showed that there were exceedingly rich membranes, concentric arrangement structures similar to rough endo-reticulum and conspiciously blocking of the formation of food vacuoles.