Introduction. The first two years of life are a critical window of opportunity for ensuring optimal child growth and development. Nutritional deficiencies during this period can lead to impaired cognitive development, compromised educational achievement, and low economic productivity. Improving infant and young child feeding (IYCF) practices in children aged 0-23 months is therefore critical to improved nutrition, health, and development. Objective. The aim of the study is to assess the prevalence of complementary feeding practice and its associated factors among mothers with children aged 6−23 months in Lasta District, Northeast Ethiopia, 2015. Methods. A community based cross-sectional study design was conducted among 476 mothers who had children aged 6-23 months in the study area. Simple random sampling technique was used to select the required sample. A face-to-face interview was done to collect data using structured questionnaire. Data were entered with EPI info version 3.5.1 and cleaning and analysis were done using SPSS version 16. Frequencies distribution and binary and multiple logistic regressions were done. Results. In this study only 56.5% of children aged 6-23 months received appropriate complementary feeding, considering timely introduction, minimum dietary diversity, and meal frequency. Exposure to public media [AOR = 2.50; 95% CI: 1.44, 4.35], occupation of mother [AOR = 9.50; 95% CI: 1.02, 14.25], mothers decision making role on how to use family income [AOR = 5.54; 95% CI: 1.19, 11.74], and use of postnatal care service [AOR = 5.98; 95% CI: 1.49, 13.96] were found to be independent predictors of complementary feeding practice. Conclusion and Recommendation. About 43.5% of mothers were not feeding their children complementary food appropriately, which would have negative implication on the health of infants and young children. There was a statistically significant association of inappropriate complementary feeding practices with mothers' occupation, postnatal care service, media exposure, and mothers' decision making role on how the money is used. Health professionals should focus on advising and counseling mothers on appropriate complementary feeding during prenatal, delivery, postnatal, and immunization services.