M. Sc. DissertationProper complementation for children under the age of five years is important for optimal health. More than a quarter of under-five children in Tanzania are malnourished with prevalence rates of stunting (29%), underweight (11.8%) and wasting (4.7%) in Singida region. A longitudinal survey was conducted to determine the effect of complementary feeding on the growth patterns of under-five children in Majiri ward, Manyoni District, Singida Region. Three hundred and sixteen mother-child pairs were selected for the baseline survey. Dietary diversity was assessed by using un-quantified validated 24-hour recall method. Anthropometric measurements were taken from children and their respective mothers at baseline, follow-up 1 and follow-up 2 visits. Data were analysed using ENA for Smart program, Excel and IBM Statistical Product for Service Solution (SPSS) program version 20. Results showed that, the mean age of initiation of complementary feeding was 3.81 ± 1.63 months. There was no significant difference (p> 0.05) in the time of initiation of complementary feeding among mothers of different education levels. Most children (84.8% n=190) met the minimum recommended dietary diversity of four food groups per day. Plant based protein was most commonly source of protein in the complementary foods. Marital status (β = 53.92; P = 0.044), maternal age (β = 34.07; P = 0.008) and maternal education levels (β = 89.99; P = 0.001) were significantly associated with time of introduction of complementary foods. Consumption of less than four food groups per day in children was associated (P < 0.05) with stunting. Underweight, stunting and wasting prevailed more in children who had complementary foods before six months. Complementing children at earlier life (less than six months) increased the odds of becoming undernourished compared to those who were complemented at latter ages (more than six months). It was concluded from this study that, children who had their complementary feeding earlier than six months were more likely to become undernourished. It was concluded from this study that, dietary diversity was not only the major factor determining under nutrition, other factors such as maternal age, education level and socio-demographic factors have strong influence on the nutritional status. It was recommended that, mothers should be educated on proper timing of complementing their children and on the importance of dietary diversity for the proper growth of their young children.