Based on the oxygen ion conductor La 2 Mo 2 O 9 , a series of Sm-doped substitutes (La 2 - x Sm x Mo 2 O 9 , x=0.1, 0.2, 0.3, 0.4) were prepared. The structure, influence of phase transition and oxygen ion conductivity were studied with DSC, HRTEM, high temperature and room temperature XRD, and complex impedance spectra. For comparison, Bi-doped substitutes (La 2 - x Bi x Mo 2 O 9 , x=0.1, 0.2, 0.3, 0.4) were also prepared and studied. Experimental results showed that the substitution of La with appropriate amount of Sm can effectively stabilize the β-form of La 2 Mo 2 O 9 at room temperature and inhibit its phase transition to α-form, and therefore presented the substitutes much higher ionic conductivity than the pristine La 2 Mo 2 O 9 . In addition, co-substitution of both La and Mo sites were achieved with Sm or Bi and W, and typical substitutes La 2 Mo 1 . 7 W 0 . 3 O 9 , La 1 . 7 Sm 0 . 3 Mo 1 . 7 W 0 . 3 O 9 and La 1 . 7 Bi 0 . 3 Mo 1 . 7 W 0 . 3 O 9 were obtained, which to some extent improved the ionic conductivity of La 2 Mo 2 O 9 , especially at lower temperatures.