Critical shear stress and threshold stream power are two important soil characteristics controlling detachment of soil particles by runoff and have been used in process-based erosion models such as WEPP, GUEST and EUROSEM. In this research, laboratory experiments were conducted in a 20 × 350 cm flume to study the effects of particle size and density on initial motion. Two contrasting soil samples, a well-aggregated forest soil and non-cohesive fluvial sand, were used to provide particles with different densities. Each sample was divided into six size classes. Flow bed in the flume was roughed according to testing area for each size class using a plate which sand particles from each size class were glued on it. The initial motion of the particles was determined by two methods. In the first method, slope was increased gradually for a given constant discharge until particles start to move from every point of the testing area. In the second method, flume slope was set to a given steepness and discharge was gradually increased until particles start to move. Three different discharges and three slopes were tested in the first and second methods, respectively. Each test replicated two times. Analysis of the data showed that the particle size and density and also their interaction significantly affect (P < 0.001) critical shear stress and threshold stream power. The critical shear stress and threshold stream power increased with increasing particle size and density, but the impact of particle density is higher on the coarser particles than the finer ones. Threshold values measured for the sand particles were about 2.3 times of those measured for soil particles in the three coarser classes, this difference decreased to about 65% (1.65 times) in the three finer classes, and even the difference between the two types of particles was not significant for the finest class (0.125–0.053).