Fig. 5 shows the dependence of the initial permeability μi at room temperature and I1 on the ionic radii of the substituted ions. The increase in μi attributes to the decrease of intragranular pores which is associated to I1 inside the bulk, ( Fig. 5) except for Gd–Li sample. To account on the decrease of μi for Gd–Li sample, we must take into consideration the distribution of intragranular pores inside bulk ferrite. It is known that the initial permeability results from reversible motion of domain walls under the effect of very small applied magnetic field, and the rotation of spins inside domains (negligible) [24]. So, as μi for Gd-sample is very small although it has small intragranular pores concentration compared to the other samples. One might consider these pores are concentrated at the walls of the domains. This makes the motion of domain walls harder inside the grains and accounts on the very small initial permeability value for Gd sample relative to the other samples. From another point of view, it is reported that Ms decreases with increasing the ionic radii of the substituted ions for Li-ferrite [11]. The decrease of μi could be explained according to Globous relation, μi α (Ms)2.(G.S) [24], accordingly the decrease of the initial permeability μi is mostly dominated by the decrease of the saturation magnetization value for Gd–Li sample. Then the effect of the decrease of magnetization overcomes the decrease of the intragranular porosity and the increase of average grain size, ( table 1). Meanwhile, the increase of μi for all other samples indicates that the dominant factor is the decrease of I1 or intragranular pores and the increase of grain size, Table 1. These two parameters overcome the magnetization effect inside domains of bulk sample. So, there is a correlation between the initial permeability, grain size and the relative intensity I1, in other words intragranular pores. It is reported that the initial permeability decreases with increasing the total pores Pt for most of published papers [9], [10] and [11]. An important point obtained by positron annihilation analysis for ferrites is the dependence of the initial permeability on the intragranular pores rather than the considering of the total pores, although it is a qualitative consideration. This novel point is very important to studying the properties of ferrites to know qualitatively the distribution of pores inside and outside the grains and the distribution of intragranular pores inside domains and at the walls of them.