Li-ion batteries hold an important place in the field of high power batteries because of their high open circuit voltage and associated high energy density. However, the safety is less satisfactory; therefore, the study of the factors that affect the safety of Li-ion batteries has much meaning to the safety design. In this paper, a set of apparatus was developed for in situ measurements, and several commercial materials including electrolyte, separator and electrode materials for Li-ion batteries were investigated by the in situ method. The results showed: 1) The electrolyte vapor pressure is influenced significantly by the component with low boiling point and increases rapidly with the increasing of temperature; 2) the shutdown of separator occurs at around 135℃ and the impedance increases approximately by two orders of magnitude; 3) carbon anode materials affect the most the volume changes of the cell, and the change for a graphite anode is much greater than that of a glassy carbon anode.