The effect of partial substitution of lanthanum to barium in hexagonal ferrite on the structure and magnetic properties of mechanically alloyed (Ba1-xLax)Fe12O19 (x=0 and 0.3) is described. Precursors of La2O3, BaCO3 and Fe2O3 were ball milled and sintered to obtain nanocrystalline particles of un-doped Ba0.6(Fe2O3) and doped (Ba0.7La0.3)Fe12O19. Phase studies in both materials indicated that (Ba0.7La0.3)Fe12O19 showed a presence of secondary Fe2O3 phase in addition to the main phase. It indicated that La substitution in M-type barium hexaferrite has a solubility limit. It is also shown that crystallite sizes of doped sample have a broaden size distribution ranging from a few nanometers to 200 nm with the mean size 37 nm, which different with un-doped sample where the distribution width and mean crystallite size were respectively slimmer and smaller. Magnetic properties of doped material showed a decrease in energy product (4.2 kJ.m-3) despite an increase in coercivity (234 kA. m-1 and 198.4 kA. m-1 respectively).