Abstract We have conducted systematic investigations of formation age, chemical compositions, and mineralogical characteristics of ferromagnesian chondrules in Yamato-81020 (CO3.05), one of the most primitive carbonaceous chondrites, to get better understanding of the origin of chemical groups of chondrites. The 26Al– 26Mg isotopic system were measured in fourteen FeO-poor (Type I), six FeO-rich (Type II) and two aluminum-rich (Al-rich) chondrules using a secondary ion mass spectrometer. Excesses of 26Mg in plagioclase (1.0–13.5‰) are resolved with sufficient precision (mostly 0.4–6.6‰ at 2 σ level) in all the chondrules studied except one. Chemical zoning of Mg and Na in plagioclase were investigated in detail in order to evaluate the applicability of 26Al– 26Mg chronometer. We conclude that the Al–Mg isotope system of the chondrules in Y-81020 have not been disturbed by parent-body metamorphism and can be used as chronometer assuming homogeneous distribution of 26Al. Assuming an initial 26Al/ 27Al ratio of 5 × 10 −5 in the early solar system, 26Al– 26Mg ages were found to be 1.7–2.5 Ma after CAI formation for Type I, 2.0–3.0 Ma for Type II and 1.9 and 2.6 Ma for Al-rich chondrules. The formation ages of ferromagnesian chondrules in Y-81020 are in good agreement with those of L and LL (type 3.0–3.1) chondrites in the literature, which indicates that common chondrules in the CO chondrite were formed contemporaneously with those in L and LL chondrites. The concurrent formation of chondrules of CO and L/LL chondrites suggests that the chemical differences between CO and L/LL chondrites might be caused by spatial separation of chondrule formation environments in the protoplanetary disk.