Surface charge of inorganic colloids in soils(brick red soil, red soil, brown yellow soil, dark brown soil, black soil) distributed in different latitudes of China was studied with the back titration method and Mehlich method. Results show that: (1) a big difference exists between the two methods, mainly because they are based on different measuring units, but anyway they reflect surface charge of inorganic colloids from different sides; and (2) either has its own advantages. When used for determination of surface charge of inorganic colloids, however, both of them have to be modified in terms of practical procedure in determination.