Hot tearing, also referred as hot cracking, is a commonly encountered solidification defect during castings. Although it has been extensively investigated for decades, most contributions are still based on qualitative characterizations. In this work, a quantitative method to investigate hot tearing has been developed, based on the measurement of contraction force. The advantage of the present evaluation system, which is mainly different from previous apparatus, is to eliminate the influence of friction between the mould and casting rod., The results obtained using this system demonstrate good repeatability and reliability. The measured contraction force has been proved to be able to evaluate the hot tearing susceptibility as a more straightforward and quantitative index. By analyzing the contraction force curve, both the initiation and propagation of hot tear can be detected. With this method, the hot tearing of Mg-Al binary alloys has been investigated. The results show that increasing the mould temperature decreases the hot tearing susceptibility, meanwhile increases the temperature of the hot tearing initiation. The contraction force curves also indicate that the liquid refilling plays an important role in the formation of hot tearing. With a lower cooling rate, the remained liquid possibly refills the initiated hot crack, and consequently partially or completely heals the cracks.