The invention relates to a process for preparing a powder formulation for inhalation for use in a dry powder inhaler, said powder formulation comprising: (A) a carrier, comprising : (a) a fraction of coarse particles of a physiologically acceptable carrier having a particle size of at least 60 micron； and optionally (b) a fraction of fine particles comprising a physiologically acceptable excipient, wherein at least 90% of all said fine particles has a volume diameter lower than 15 micron, (B) micronized particles of glycopyrronium bromide, an inhaled corticosteroid (ICS), and optionally a long-acting O2 -agonist (LABA), as active ingredients, wherein said process comprises: (i) preparing by co-milling microparticles consisting of glycopyrronium bromide and a first part of the ICS in a ratio ranging from 80:20 to 70:30 by weight, wherein the volume diameter of said microparticles is not higher than 15 micron； (ii) mixing the carrier, the remaining part of said ICS, and optionally the long-acting O2 -agonist, to obtain a first mixture； and (iii) adding the co-milled microparticles obtained in step (i) to the first mixture obtained in step (ii), to obtain a second, final, mixture.
