The calculation method of forming limit of sheet metals based on M-K instability theory is proposed,and the method is applicable to different yield criterions and hardening models.The forming limit dia-grams of AA5754-O,AA6111-T4 aluminum alloy sheet and DP steel sheet under combined loading paths are obtained based on mixed hardening model with YLD2000-2D yield criterion proposed by Barlat in 2003 and L-C nonlinear kinematic hardening model proposed by Lemaitre and Chaboche.The results show that the forming limit diagram made up of limit strain(FLD-strain) is evidently influenced by the loading path.The forming limit diagram made up of limit stress(FLD-stress) is also influenced by loading path and it is not an only curve,which differs from the conventional view.The degree of the influence of loading path on FLD-stress is related with pre-strain.The larger the pre-strain is,the greater the influence of loading path on FLD-stress will be.The change of FLD-stress is small only when pre-strain is small.In addition,the hardening behavior of the material will influence the path-dependence of FLD-stress:The larger the proportion of kinematic hardening in the whole hard-ening is,namely the more obvious Bauschinger effect of the material,the greater the influence of loading path on FLD-stress will be. The calculation method of forming limit of sheet metals based on MK instability theory is proposed, and the method is applicable to different yield criterions and hardening models. The forming limit dia-grams of AA5754-O, AA6111-T4 aluminum alloy sheet and DP steel sheet under combined loading paths are obtained based on mixed hardening model with YLD2000-2D yield criterion proposed by Barlat in 2003 and LC nonlinear kinematic hardening model proposed by Lemaitre and Chaboche. The results show that the forming limit diagram made up of limit strain (FLD -strain) is evidently influenced by the loading path. The forming limit diagram makes up of limit stress (FLD-stress) is also influenced by loading path and it is not an only curve, which differs from the conventional view.The degree of the influence of loading path on FLD-stress is related with pre-strain is, the greater the influence of loading path on FLD-stress will be the change of FLD-stress is small only when pre- strain is small.In addition, the hardening behavior of the material will influence the path-dependence of FLD-stress: The larger the proportion of kinematic hardening in the whole hard-ening is, the more obvious Bauschinger effect of the material, the greater the influence of loading path on FLD-stress will be.