Two additive-induced isomeric aluminoborates, (CH(3)NH(3))(2)(H(2)O)(2)[Al(B(5)O(10))] (1) and (CH(3)NH(3))(H(2)O)[Al(B(5)O(10))] (2), have been obtained by using small methylamine molecules as structure directing agents (SDA) under solvothermal conditions and characterized by elemental analysis, IR, TGA, powder X-ray diffraction, single-crystal X-ray diffraction, NLO determination, UV-vis spectral investigation, electronic structure calculation and ion exchange. Different arrangements of B(5)O(10) units and AlO(4) tetrahedra results in two different structures with two pairs of helical channels along [100] and [010] directions, respectively. Interestingly, the two frameworks with noncentrosymmetric and chiral space groups separately exhibit the same diamond topologies and display good second harmonic generation efficiencies of about 2.0 times the value of KDP.