1. A method of removing a blocking artifact from an image in a loop filtering system, the method comprising: determining whether a first block and an adjacent second block have non-zero transform coefficient information; selecting a filtering strength value based on results of the determining step; obtaining quantization information of the first block and the second block; determining a filtering coefficient based on a value obtained from a table using the quantization information; and filtering four successive pixels based on the filtering coefficient and the filtering strength, two pixels of the four successive pixels being in the first block and two other pixels of the four successive pixels being in the second block.