The present invention relates to a unitary absorbent structure and method thereof wherein said unitary absorbent structure comprises an absorbent core (5) and/or an acquisition and dispersion layers (2) (3) and comprising at least one non-woven fibrous substrate layer (23) having a void volume suitable to be penetrated by super absorbent particles. The super absorbent particles are dispersed in the substrate layer (23) according to a size distribution gradient by vacuum (8′) and vibration along the depth direction or z-direction of said absorbent core (5) and/or acquisition (2) and dispersion (3) layers, the smaller particles are placed on the bodyside of the absorbent articles and the larger particles are located on the opposite side of the absorbent articles.