Solidification cracking is one of the defects of casting. In order to increase productivity of DC casting and various shape casting processes, predicting the occurrence of solidification cracking by using CAE (Computer Aided Engineering) should be essential. Therefore, it is necessary to obtain mechanical properties of solid-liquid coexistence alloys. However, in the previous reports, flat distribution of temperature in the gage length was not ensured. In this study, tensile test device for a semi-liquid Al–Mg alloy with 2°C of the temperature distribution in the gage length (Max. 10 mm) was developed. Tensile strain was measured by using in-situ observation of marker on the surface of the specimen. As the result, stress-strain curves and fracture strain in various temperatures on the semi-liquid Al–4.3mass%Mg alloy were measured. It is also found that tensile strain tends to be higher at neighborhood of the crack. Thus, strain depends on the gage length. From the observation of the fracture surface and estimation of Scheil-Gulliver micro-segregation, microstructure of the specimen were supposed to change due to the heating process.