Abstract: A false mode>100 μm often occurs in laser grain-size measurement. The causes of the false mode may be that the water amount <800 mL leads to a high liquid viscosity and in turn stirs more bubbles. Another cause may be that larger quantity of a sample may induce a larger shading level of light (>13%) and a higher liquid viscosity in turn may release bubbles with difficulty. An inappropriate pump speed or an ultrasonic dispersion time shorter than 2.5 min also may result in false modes appearing. Several principles to avoid false modes are concluded. The shading level of light (obstruct light) should be 8%~13%. The amount of samples is determined according to their types. The appropriate quantity is~0.1 g for clay samples, 0.2 g for loess and fine silty sand samples, 0.4 g for coarse silty sand samples, 0.6 g for fine sand samples and 1.0 g for sand samples in order to keep the light shading level of 8%~12%. The water amount of~1 000 mL and the ultrasonic dispersion time of>2.5 min before measuring are necessary to eliminate false modes of grain size distribution. For clay or fine silty sand (median size<10 μm), false modes may occur even though water amount is more than 1 000 mL. In the case, false modes can be identified and eliminated by changing stir speed. The mode whose median size is>100 μm and does not occur in all conditions may be a false mode. The false mode may disappear when the stir speed is reduced to 2 000 for a sample with lower light shading level. The false mode may disappear when the stir speed is increased to 3 200 for a sample with higher light shading level.