Experiments on initiation of motion and rate of bed load transport of different fractions in case of nonuniform sediments are reported. Analysis of these data as well as those from earlier studies by other investigators has shown some limitations of the available relationships for critical tractive stress (CTS) of sediment mixtures. Further analysis of the data has resulted in a new relationship for CTS in terms of the size of the particular fraction in question (di), the geometric mean size (dg) and the geometric standard deviation (σg) of the mixture. An empirical relationship has been obtained for the critical tractive stress of the sizedσdefined asdgσg.