Structural modification of three dimensional (3D) materials for the application of dielectric loss-based microwave absorbing materials (MAMs) usually relies on intricate synthesis process and can pose challenges in terms of scalability and mass production for practical application. In this work, we reported a successful attempt in modifying the 3D structure of mesoporous lanthanum oxide (La2O3) for effective broadband MAMs candidate via simple co-precipitation process. The inclusion of cetyl-trimethylammonium bromide (CTAB) and hydrothermal aging treatment result in a significant transformation of La2O3 particles from their original polygonal form to a 3D coral-like and nano needle-like structure. The utilization of CTAB and hydrothermal aging results in the increase of surface area and a two-fold increase in pore volume of the resulting La2O3. Due to its unique 3D structure, the 3D coral-like and nano needle-like La2O3 materials possess a broadband electromagnetic (EM) wave absorption characteristic with the effective absorption bandwidth (EAB) covering the C-band frequency range. Specifically, in the La2O3 C-H sample (with CTAB – with hydrothermal), it exhibits strong EM wave absorption with a reflection loss (RL) value of –33.07 dB which equals to 99.95% EM wave absorption at a thickness of only 1.50 mm. The detailed analysis of EM wave absorption properties reveals that the improvement of La2O3 materials to attenuate EM wave energy arises from the dielectric loss phenomenon, the enhanced interfacial polarization, multiple reflections mechanism, and conduction loss mechanism induced by the 3D structural formation of the La2O3 structure. This work proposes a novel and efficient approach in synthesizing and modifying 3D materials for effective broadband EM wave absorption.