A method of decoding a motion vector, the method comprising: determining whether motion vectors of spatial prediction units that are spatially collocated to a current prediction unit and motion vectors of temporal prediction units that are temporally collocated to the current prediction unit are available; determining the available motion vectors as motion vector predictor candidates; characterized by: when the number of the motion vector predictor candidates is less than a predetermined number 'n', where n is an integer, generating at least one combined bi-directional motion vector by combining an available motion vector, from among the available motion vectors, which indicates a block of picture in reference picture list 0 and an available motion vector, from among the available motion vectors, which indicates a block of picture in reference picture list 1; adding the at least one combined bi-directional motion vector to the motion vector predictor candidates; when the number of the motion vector predictor candidates is less than the predetermined number 'n', adding a motion vector having a predetermined component value to the motion vector predictor candidates so that a total number of motion vector predictor candidates is the predetermined number n, wherein the predetermined component value is (0, 0); obtaining information indicating a motion vector predictor among the motion vector predictor candidates from a bitstream; and obtaining the motion vector predictor of the current prediction unit based on the information, wherein the spatial prediction units comprises a spatial prediction unit located on a lower left side of the current prediction unit. determining whether motion vectors of spatial prediction units that are spatially collocated to a current prediction unit and motion vectors of temporal prediction units that are temporally collocated to the current prediction unit are available; determining the available motion vectors as motion vector predictor candidates; characterized by: when the number of the motion vector predictor candidates is less than a predetermined number 'n', where n is an integer, generating at least one combined bi-directional motion vector by combining an available motion vector, from among the available motion vectors, which indicates a block of picture in reference picture list 0 and an available motion vector, from among the available motion vectors, which indicates a block of picture in reference picture list 1; adding the at least one combined bi-directional motion vector to the motion vector predictor candidates; when the number of the motion vector predictor candidates is less than the predetermined number 'n', adding a motion vector having a predetermined component value to the motion vector predictor candidates so that a total number of motion vector predictor candidates is the predetermined number n, wherein the predetermined component value is (0, 0); obtaining information indicating a motion vector predictor among the motion vector predictor candidates from a bitstream; and obtaining the motion vector predictor of the current prediction unit based on the information, wherein the spatial prediction units comprises a spatial prediction unit located on a lower left side of the current prediction unit. when the number of the motion vector predictor candidates is less than a predetermined number 'n', where n is an integer, generating at least one combined bi-directional motion vector by combining an available motion vector, from among the available motion vectors, which indicates a block of picture in reference picture list 0 and an available motion vector, from among the available motion vectors, which indicates a block of picture in reference picture list 1; adding the at least one combined bi-directional motion vector to the motion vector predictor candidates; when the number of the motion vector predictor candidates is less than the predetermined number 'n', adding a motion vector having a predetermined component value to the motion vector predictor candidates so that a total number of motion vector predictor candidates is the predetermined number n, wherein the predetermined component value is (0, 0); obtaining information indicating a motion vector predictor among the motion vector predictor candidates from a bitstream; and obtaining the motion vector predictor of the current prediction unit based on the information, wherein the spatial prediction units comprises a spatial prediction unit located on a lower left side of the current prediction unit.