The electrification of fine particles in a gas-solids pipe flow is investigated theoretically and experimentally. Flowing particles are charged through their collisions with a pipe wall. When a part of the pipe is electrically isolated and grounded, the charge transferred from the particles to the wall is detected as an electric current from the wall. The current generated and the electric charge of fine particles(fly-ash: D_p50=3.4 and 12μm) and relatively large particles (quartz sandD_p50=320μm are measured. The relationship between the current per unit powder flow rate I/W_p and charge-to-mass ratio (q/m_p)₀ is represented by a linear equation irrespective of particle diameter as long as the mass flow ratio is below 0.0058. However, the absolute value of I/W_p of fine particles decreases with increasing mass flow ratio m when m exceeds 0.011. It is found that the current I/W_p is represented by a quadratic equation of(q/m_p)₀. The fact is well explained by introducing the concept of electrification efficiency.