A method and apparatus are directed to applying deblocking filter to block boundaries in a video coding system including an IntraBC (Intra-block copy) mode. For at least one combination of the two neighboring reconstructed blocks including at least one IntraBC coded block in the two neighboring reconstructed blocks, a boundary filter strength equal to one or zero is selected based on coding parameters associated with the two neighboring reconstructed blocks. Deblocking filter is then applied to neighboring samples of the two neighboring reconstructed blocks around the block boundary using the boundary filter strength selected. For example, when the two neighboring reconstructed blocks correspond to one IntraBC coded block and one Inter coded block, the boundary filter strength is set to one.