Method and apparatus for constrained de-blocking filter are disclosed. According to one method, if a CU is partitioned into multiple sub-CUs, the de-blocking process is also applied to the sub-block boundaries inside the current filtered-reconstructed block. According to another method, if first reference samples used for the de-blocking process of a first boundary are to be modified by the de-blocking process of a second boundary, the first reference samples are replaced by padding samples that are not to be modified by the de-blocking process of the second boundary. According to yet another method, the de-blocking process is applied to a reconstructed block corresponding to a current block to result in a current filtered-reconstructed block regardless whether a boundary of the current block corresponds to an 8×8 sample grid boundaries.