In dry powder process, particles collide against each other and onto wall, and normally, electrostatic charge is generated. Such a process is called trioelectrification or tribo-/contact-/impact-charging of particles. In this paper, the amount of maximum (or saturated or equilibrium) charge on a particle in a powder flow of a pneumatic conveyer with a metal pipe was discussed. A theoretical discussion involving the space charge effect with the basis of 'charge relaxation model,' in which the maximum charge is limited to cause gas break down, is shown. The result showed two regions in which the space charge does not affect on, and dominates the amount of the charge. The charge was proportional to the particle size raised to the power 1.5 in the region of no space charge effect, whilst power 3 in the other region of the space charge dominant. Empirically a well approximated analytical formula was found, for all over the range including these two regions. The maximum charge as a function of particle size, pipe size, and volume fraction of the powder, was given as: q/C = 1.10 x 10(-4) (d(p)/m)(3)/root{phi(D/m)}(2)+{17.1(d(p)/m)(1.5)}. A comparison showed that the theory gives a certain overestimation to the data cited from literatures. Additional discussions were given in details. (C) 2010 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.