The crystallographic structural and magnetic properties of polycrystalline La0.7Sr0.3Mn1–xCoxO3 ( $x=0.4$ , 0.5, 0.6, 1.0) have been systematically studied in detail by neutron powder diffraction and magnetic measurements. The introduction of Co dopant to the Mn network has decreased the bandwidth ( $W$ ) and the overlap between the O- $2p$ and the Mn- $3d$ orbitals. In addition, it in turn weakened the fragile double exchange coupling of Mn3+–O–Mn4+ and reduced the magnetization $M$ as well as magnetic ordering temperature $T_ {c}$ . The magnetic measurements indicate the presence of mixed magnetic phases in this system. When the temperature is above $T_{c}$ , the jump of field-dependent magnetization disappeared, which is related to the competition of ferromagnetic and antiferromagnetic interactions. The average atom magnetization extracted from $M$ – $H$ curve at 5 K using the approach-to-saturation method, is $\sim 1.70~\mu _{\mathrm {B}}$ /Co for La0.7Sr0.3CoO3. This value approaches to the neutron refined data 1.93(8) $\mu _{\mathrm {B}}$ . However, as for $x = 0.4$ , $x = 0.5$ and 0.6 samples, the calculated data from magnetization at 5 T are all smaller than the refined one, due to the existence of spin-glass-like phase. The weak magnetic peaks are responsible for the small magnetic moment data ( $0.48~\mu _{\mathrm {B}}$ for LSMO–Co0.5 and $0.65~\mu _{\mathrm {B}}$ for LSMO–Co0.6). These results indicate the competing magnetic interactions in the system of La0.7Sr0.3Mn1–xCoxO3 compounds.