A method of regenerating a lean NOx trap (LNT) of an exhaust purification system provided with the LNT and a selective catalytic reduction (SCR) catalyst may include: determining whether a regeneration release condition of the LNT is satisfied; determining whether a regeneration demand condition of the LNT is satisfied; and performing regeneration of the LNT if the regeneration release condition of the LNT and the regeneration demand condition of the LNT are satisfied. In particular, the regeneration demand condition of the LNT is satisfied if a NOx adsorption in the LNT is larger than or equal to a threshold NOx adsorption, and the threshold NOx adsorption is set to a minimum value of a target NOx adsorption in the LNT which changes according to a maximum NOx adsorption in the LNT in cold starting and a NOx purification efficiency of the SCR catalyst.