Introduction: The quality of feeding in early childhood depends on nutritional knowledge of mothers. Improving maternal nutrition knowledge is therefore pivotal towards promoting effective infants’ feeding behaviour. Objective: Study assessed mothers’ knowledge about complementary feeding and complementary feeding practices. Study also assessed effectiveness of Behaviour Change Communication on mothers’ knowledge and complementary feeding practices, compared the nutritional status of infants whose mothers received intervention to infants of mothers in the control group. Materials and Methods: A quasi-experimental study conducted among 204 mothers in rural and urban local government areas, South-west Nigeria between March and September, 2019. Sample size was estimated using formula for comparison of two proportions, eligible mothers were selected through multistage sampling technique. Study was conducted in three phases: pre-intervention phase, intervention and post intervention phases. Intervention: Selected mothers were randomly assigned into intervention and control groups. Mothers in the intervention group received Behavior Change Communication on complementary feeding and were followed up for six months. Data Analysis: Data was analyzed using SPSS software version 25, Chi-square and repeated analysis of variance evaluated effectiveness of intervention, level of significance was < 0.05. Results: BCC improved mothers’ knowledge about complementary feeding by 31.9% (χ2 = 21.62; p = 0.001), meal frequency by 16% (χ2 = 5.88, p = 0.01), dietary diversity by 19.6% (χ2 = 7.44, p = 0.01), minimum acceptable diet by 20.6% (χ2 = 13.09; p =0.01). Intervention also reduced under-weight by 14% (χ2 = 0.69, p = 0.01; (F(1,191) =275.34; p = 0.04) among intervention group. Conclusion: Effective nutritional intervention for mothers towards improving nutritional status of children should incorporate appropriate behaviour change approach. This approach is capable of improving nutritional status of infants and children and consequently reduce malnutrition and related complications in early childhood.