A method for a video encoder includes steps of: receiving a series of input frames; performing at least one of intra-prediction and inter-prediction on the series of input frames for generating at least one of an intra frame (I-frame), a predictive frame (P-frame), and a key predictive frame (key P-frame); generating an encoded bitstream with respect to a first input frame determined as the I-frame among the series of input frames; and performing inter-prediction on a second input frame determined as a key P-frame among the series of input frames according to a first reference frame which is derived from the encoded bitstream with respect to the first input frame.