The substances SrM3P4O14 (M = Mn, Fe, Co or Ni) have the same crystal structure. However,the magnetism of SrMn3P4O14 differs qualitatively from that of the other substances. We determinedthe magnetic structures of SrM3P4O14 (M = Mn or Co) by using neutron powder diffractionmeasurements. The spin system in SrMn3P4O14 can be regarded as a trimerized spin chain, whichis consistent with the spin system inferred from the crystal structure and the appearance of the 1/3quantum magnetization plateau. The spin system in SrCo3P4O14 is not a trimerized spin chain.