Mg-Zn-Mn alloy has high hot cracking tendency (HCT), but few researches focus on its hot cracking behavior and mechanism. The effect of Mn on the HCT of Mg-6.5Zn-xMn alloys was studied by the designed equipment which can measure and record the subtle changes of temperature, shrinkage displacement and shrinkage stress during solidification in this study. The results indicate that the larger the maximum contract rate (v_max) and the stress accumulating coefficient (k), which are put forward to evaluate HCT, the higher the HCT is, and there is higher HTC when v_max or k presents at high fraction of solid. The v_max of Mg-6.5Zn-xMn alloy increases with the increase of Mn content, however its position move towards to lower fraction of solid, and the k reaches the maximum value and presents at high fraction of solid at 0.35%Mn, which means the greatest HCT in this composition. The hot cracks of these alloys initiated and propagated at final stage of solidification (with higher fraction of solid), and the intergranular feeding channels could be observed. The thicker the liquid film around grains formed by the low melting point phases and the finer the grains, the less the HCT of the alloy is. After dendritic separation, interdendritic bridging formed by the jointing of dendrite arms could enhance the adhesive force between grains at final stage of solidification. However, the break of interdendritic bridging due to the hindrance to grain contraction would result in the hot cracks.