It has been reported that human cancer cells are more sensitive to high temperatures than normal human cells, and that cell proliferation and viability are affected by the temperature environment. In this study, we proceeded further, and turning our attention to the close relationship between cell morphology and temperature, used two human cancer cell lines and two normal cell strains to investigate how intracellular fine structure changes in a high temperature environment. The results showed that 1) both of the human cancer cell lines were more sensitive to high temperature than the normal human cell strains, and a difference between the temperature sensitivity of the human cancer cell lines was also confirmed. 2) There is no clear difference between the manner in which normal human cells and malignant human cells are affected by hyperthermia. 3) Among other cell structures, effects on the membrane system were observed as early changes in cell structure. The mitochondria were particularly affected, followed by the rER. 4) Changes in the nucleoplasm, as well as the nuclear membrane (inner membrane), and then the intranuclear chromatin, etc., were observed as late changes. 5) Changes in mitochondria were observed in the early stage, but temporarily tended to recover, and were then fatally affected again in the late stage. We discuss the relationship between cell proliferation, cell viability, and cell ultrastructure based on the above results.