Undernutrition and hidden hunger threaten the survival, growth, and development of children, young people, economies, and nations. Inappropriate complementary feeding practice due to poor maternal knowledge and awareness in combination with low income and infectious disease is the contributing factor for child undernutrition. Hence, this study was aimed at determining the effect of nutrition education on improving the knowledge and practice of complementary feeding of the mothers with 6- to 23-month old children in daycare centers of Hawassa Town, Southern Ethiopia. An institution-based randomized control trial design was employed. Daycare centers were randomly allocated for the intervention group and the control group. Among the total daycare centers in the town, five were assigned to receive nutrition education and the rest five for the control group (CG). The simple random sampling technique used to select individual participants from each daycare center. Two hundred (200) mother-child pairs (100 for each group) were recruited. Sociodemographic and economic variables were collected by the structured questionnaire. Knowledge of appropriate complementary feeding was assessed by seven knowledge questions. Appropriate complementary feeding practice was assessed by adapting Alive and Thrive Infant and Young Child Feeding (IYCF) practice guidelines. Nutrition education was given for four consecutive months by using Alive and Thrive IYCF guidelines. Data were analyzed by the SPSS software program version 20. The chi-squared test was used to test the significant differences in the proportion of good knowledge and good practice of complementary feeding and good dietary diversity between two groups. The independent t test was used to test the significant differences in mean dietary diversity between two groups. At 95% confidence interval, p<0.05 was considered statistically significant. The results revealed that the proportion of mothers with good knowledge of appropriate complementary feeding was increased from 59% at pretest to 96% at posttest and the appropriate complementary feeding practice was improved from 54% at pretest to 86% at posttest in IG. There was no change in the knowledge and practice of complementary feeding practice in CG after four months. The proportion of mothers with good complementary knowledge was 54% both at pretest and at posttest and good complementary feeding practice was 51% and 52% at pre- and posttest in CG, respectively. There was no significant difference (p>0.05) on complementary feeding knowledge and practice between two groups at pretest, while the difference was highly significant (p<0.05) at the posttest. In conclusion, providing nutrition education improved the appropriate complementary feeding knowledge and practice of mothers. In recommendation, government and other partners working on sustainable child nutrition reduction should focus on the nutrition education to improve the knowledge and appropriate complementary feeding practice including daycare centers.