A neutron powder diffraction (NPD) study on the crystal and magnetic structure of a crushed La Sr MnO (x ≈ 1/8) single crystal has been performed. The sample belongs to orthorhombic (Pnma, O) above the Jahn-Teller (JT) transition temperature (T ) and monoclinic (P12 /c1, M')in the JT regime. We have also refined the NPD data below the charge/orbital ordering (CO/OO) temperature (T ) with a monoclinic (P12 /c1, M) model because the experimental resolution was insufficient to clearly identify a triclinic (P\bar {1}) structure. The refined lattice parameters show an obvious breathing-mode distortion between T and T , accompanied by a large deviation of the monoclinic angle β from 90°, signifying a very strong cooperative JT distortion. A ferromagnetic (FM) moment of 3.43(5)μ /Mn besides an A-type antiferromagnetic (A-AFM) moment of 0.54(2) μ /Mn is directed mainly along the b axis in P12 /c1 symmetry at 5 K. With increasing temperature, the A-AFM domains transform into FM ones above ~100 K and the FM spin orientation turns from the b to the c axis in crystallographic b-c plane below T = 187(1) K. The magnetization measurements show typical anomalies around T and T . The measured saturation moment of 3.9(1)μ /Mn at 70 kOe and 5 K is well consistent with the sum 3.97(5)μ /Mn of the refined FM and A-AFM moments at 5 K, implying the A-AFM spins are aligned in field direction at 70 kOe. The applied magnetic field can affect the paramagnetic insulating (PMI) state in the range of magnetic polarons. Based on the size of JT distortion and the bond-valence sums (BVS's), the CO/OO phenomenon is being discussed. © 2009 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.