A method of processing video data of slices in a video coding system, wherein video data of a color component in each slice is partitioned into Coding Blocks, CBs, according to a combined partitioning method, and the combined partitioning method partitions the video data first according to a first partitioning method and then according to a second partitioning method, comprising: receiving input data associated with a current CB in a current slice; determining a depth flag syntax element, wherein said depth flag syntax element specifies a difference in depth values between a Coding Tree Block, CTB, size and a minimum block size for QP information signaling; calculating a combined depth of the current CB by using a weighted sum of a first partitioning depth and a second partitioning depth of the current CB, wherein the first partitioning depth corresponds to the first partitioning method and the second partitioning depth corresponds to the second partitioning method; and comparing said combined depth of the current CB with said depth flag syntax element; wherein the QP information signaling for the current CB in the current slice is conditioned by said comparing result of depths, and the method further comprises: determining and incorporating QP information for the current CB in a video bitstream or parsing the QP information for the current CB from the video bitstream if said combined depth of the current CB is smaller than or equal to said depth flag syntax element; and quantizing a residual signal of the current CB or recovering a quantized residual signal of the current CB according to the QP information for the current CB; wherein the color component carried in the current CB is a chroma component, and the QP information for the current CB is determined by reusing QP information for a co-located luma block of the current CB if block partitioning structures for a luma component and the chroma component are separately determined. receiving input data associated with a current CB in a current slice; determining a depth flag syntax element, wherein said depth flag syntax element specifies a difference in depth values between a Coding Tree Block, CTB, size and a minimum block size for QP information signaling; calculating a combined depth of the current CB by using a weighted sum of a first partitioning depth and a second partitioning depth of the current CB, wherein the first partitioning depth corresponds to the first partitioning method and the second partitioning depth corresponds to the second partitioning method; and comparing said combined depth of the current CB with said depth flag syntax element; wherein the QP information signaling for the current CB in the current slice is conditioned by said comparing result of depths, and the method further comprises: determining and incorporating QP information for the current CB in a video bitstream or parsing the QP information for the current CB from the video bitstream if said combined depth of the current CB is smaller than or equal to said depth flag syntax element; and quantizing a residual signal of the current CB or recovering a quantized residual signal of the current CB according to the QP information for the current CB; wherein the color component carried in the current CB is a chroma component, and the QP information for the current CB is determined by reusing QP information for a co-located luma block of the current CB if block partitioning structures for a luma component and the chroma component are separately determined. determining and incorporating QP information for the current CB in a video bitstream or parsing the QP information for the current CB from the video bitstream if said combined depth of the current CB is smaller than or equal to said depth flag syntax element; and quantizing a residual signal of the current CB or recovering a quantized residual signal of the current CB according to the QP information for the current CB; wherein the color component carried in the current CB is a chroma component, and the QP information for the current CB is determined by reusing QP information for a co-located luma block of the current CB if block partitioning structures for a luma component and the chroma component are separately determined.