Here, phase transformation and electrochemical characteristics of non-stoichiometry La4MgNix (x = 16, 17 and 18) hydrogen storage alloys were studied. It is found that after annealed at 1223 K for 24 h, the minor AB(3) and AB(5) phases in La4MgNi16 alloy transform into A(2)B(7) phase by a peritectic reaction and the La4MgNi16 alloy shows a A(2)B(7) single phase structure. Double phase structures of A(2)B(7)/A(5)B(19) are obtained in La4MgNi17 and La4MgNi18 alloys after annealed at the same condition. The abundance of A(5)B(19) phase increases as x increases, indicating the increasing x value contributes to the formation of A(5)B(19) phases. Electrochemical studies show that the maximum discharge capacity and capacity retention at the 100 th charge/discharge cycles (S-100) of A(2)B(7) single phase La4MgNi16 alloy is 373 mAh g(-1) and 78.4%, respectively. The appearance of A(5)B(19) (minor) phase in the La(4)MgNi(17 )alloy makes a remarkable improvement in the discharge capacity from 373 mAh g(-1) to 388.8 mAh g(-1), as well as the S-100 from 78.4% to 90.1%. It is believed that the La-Mg-Ni-based alloy with the A(2)B(7) (main)/A(5)B(19) (minor) phase structure possesses the good overall electrochemical properties and is applicable to the high-power and long-cycle life negative electrode material for nickel metal hydride batteries. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.