Pepper blight in red pepper plants is a lethal disease by the oomycete Phytophthora capsici that secrets cell wall degrading enzymes and polygalacturonase (PGs) that are hydrolase catalyzing the hydrolysis of pectin. In this experiment we conducted to observe contents of cations and anions and changes of cell wall structures of the stems of the infected red pepper plants by P. capsici. To do this, we inoculated zoospores of P. capsici using syringe with 21G needle on peeled stem of the red pepper plants. Stem cell wall structures of normal and infected pepper plants were identified by using optical microscope. Also protein contents and PGs activities by P. capsici and cations and anions of the stems of the normal and infected red pepper plants were measured. The results showed that any hyphae of P. capsici and destructed cell wall structures were not found in the normal plants while some hyphae of P. capsici and destroyed cell walls were found in the pepper plants showing symptoms of pepper blight. However, the hyphae of P. capsici were found but broken cell walls were not found in the red pepper plants that did not show any symptoms of pepper blight. The cations from normal and infected plants were in the same order of K > Ca > Mg approximate to Na > Al > Fe. K, Ca, and Mg were much higher than those of normal red pepper plants. However, Ca was approximately twice as much as of that of normal ones. Anions from normal and infected plants were in the same order of Cl- > NO3-, > PO43- > SO42-. But NO3--N and SO42--S from the infected plants were almost 7.3 and 1.75 times of those from normal ones. Therefore, we assumed that K, Ca, NO, and SO42- could cause pepper blight on the red pepper plants.