Abstract In this paper we present a modified coprecipitation method to obtain single-phase non-stoichiometric M-type hexaferrite with barium surplus Ba 1+ x CoTiFe 10 O 19+ x ( x = 0.00 , 0.05, 0.10, 0.15, 0.20). X-ray analysis confirmed that the crystal structure of Ba 1+ x CoTiFe 10 O 19+ x did not collapse as the x increased in the range 0–0.20. The inconsonant variations of a -axis and c -axis lengths indicate that the excess barium is in the large holes along a -axis of Ba 1+ x CoTiFe 10 O 19+ x . Crystal growth and crystal shape transformation from plane to uniaxial associated with the x value were observed with a scanning electron microscope and transmission electron microscope. The dependence of grain size on the barium surplus ( x ) shows that the magnetic domain structure of Ba 1+ x CoTiFe 10 O 19+ x can be easily transformed from single domain to multidomain by changing the x value. Magnetic measurements show that the powders of Ba 1+ x CoTiFe 10 O 19+ x for x = 0.05 have a maximum coercivity value of H c = 566.22 Oe and a maximum remanent magnetization value of σ r = 19.97 emu / g , both have been ascribed to the pinning effects of excess barium and more grain boundaries on the coherent magnetization rotation reversal mechanism of randomly packed single-domain particle ranging from 150 to 300 nm. Specific saturation magnetization values ( σ s ) of Ba 1+ x CoTiFe 10 O 19+ x are unchanged in the x range 0–0.05 but decreased monotonously in the x range 0.05–0.20, and this may be explained in terms of the dilution effect of excess barium on the magnetic Fe 3+ cations due to the increase of cell volume. The dilution effect and the magnetization reversal mechanism by the domain wall motion of randomly packed multi-domain structure particles result in the continuous reduction of coercivity and remanent magnetization values of Ba 1+ x CoTiFe 10 O 19+ x as the x value increases in the range 0.05–0.20.