The purpose of this study was to ascertain effect of the models of diffusion and osmosis on students` understanding of the concepts and on scientific attitude. Two classes from two schools got different type of instruction, with the one using the models in i1e experimental setting, but the other using OHP film only in the expository teaching. A multiple-choice type test and a scientific attitude inventory were administered twice, about one week before and after instruction. The result showed that the instruction with the models was significantly more effective than the instruction without the models on the conceptual understanding. Both groups did show, however, low level of understanding of osmosis in a new context (the red blood cell in the hypertonic solution). While about 50 percent of the model group students could apply the concept, only 7 percent of the class without the n~del could apply it. Even though there was no significant, difference in the scientific attitude between the two groups, a significant increase in the interest of biology was shown in both groups after instruction. The result also showed that teachers could lead students to have partial or incorrect understandings by focusing students` attention on a part of the phenomena, anti by giving incorrect explanations to students.