For the investigation of the performance of in-situ combustion as an FOR method by means of mathematical simulation, physical and chemical properties of crude oil must be expressed mathematically. First of all, combustion characteristics of a crude oil-silica sand system was experimentally obtained using a gravimetric technique. Resultant equations of high temperature burning rate and thermal decom-position rate for Ishinazaka crude oil (18.2°API) are Eqs. (1) and (2) in the text, respectively. In our mathematical Model to be presented in the next report, a crude oil is assumed to be a mixture of two hypothetical components, one of which can represent the lowest gravity component and the other the highest gravity component of the crude oil. Their fraction is determined to express the gravity of the oil, and other physical properties are expressed as functions of temperature, pressure, and gravity of the oil. Then the saturated vapor pressure of oil and the gravity of vaporized oil were experimentally determined. These are shown, respectively, as Eqs. (3) and (4) in the text.