Effects of In, Zn, and Si impurities on the mechanical behavior of GaAs are investigated. Experimental results are interpreted in terms of the impurity effect on the dislocation velocity and of dislocation locking due to impurities which have been clarified by previous experiments. It is shown that in the temperature range lower than about 600 °C the impurity effect on the dislocation mobility in glide motion plays a dominant role in determining the mechanical strength, while in the higher temperature range dislocation locking by impurities controls the strength. Thus, Si impurity is the most effective in enhancing the strength in the low‐temperature range while In impurity is so in the high‐temperature range. The dislocation processes which take place during plastic deformation of any impurity‐doped GaAs crystals in the low‐temperature range are essentially the same as those taking place in highly pure crystals of other kinds of semiconductors such as Ge and Si.