Errors of saturation magnetization and magnetocrystalline anisotropy constant determined by magnetization curve of magnetically aligned unixial power were analyzed. In case of alignment factor ${Theta}_0=10{circ}$, magnetic constant errors of $Nd_2Fe_{14}B$ were calculated to be error of $M_S{ isingdotseq}1{\%};and;error;of;K_1{ isingdotseq}13;{\%}$, respectively, and magnetic constant errors of Ba-ferrite were calculated to be error of $M_S{ isingdotseq}1{\%};and;error;of;K_1{ isingdotseq}17;{\%}$. In this method, $M_s$ was found to be determined with high accuracy. High alignment is desirable for high accuracy.