Straw return has been strongly recommended in China, whereas applying biochar into soil is considered to provide more benefits for agriculture as well as the environment.  In this study, a five-year (2011−2015) field experiment was conducted to evaluate the effects of uncharred maize straw amendment (MS) and charred maize straw amendment (charred MS) on organic carbon (C) contents in bulk soil and in various soil aggregate-size and density fractions.  Compared to no amendment (CK), the bulk soil organic C content significantly improved by 9.30% for MS and by 23.4% for charred MS.  Uncharred and charred maize straw applied annually at a consistent equal-C dosage resulted in 19.7 and 58.2% organic C sequestration efficiency in soil, respectively, after the five years of the field experiment.  The percentages of macroaggregates (>0.25 mm) and occluded microaggregates (0.25−0.053 mm) obviously increased by 7.73 and 18.1% for MS and by 10.7 and 19.6% for charred MS, respectively.  Moreover, significant incremental increases of 19.4 and 35.0% in macroaggregate-associated organic C occurred in MS and charred MS, respectively.  The occluded microaggregates associated organic C significantly increased by 21.7% for MS and 25.1% for charred MS.  Mineral-associated organic C (<0.053 mm) inside the macroaggregates and the occluded microaggregates obviously improved by 24.7 and 33.3% for MS and by 18.4 and 44.9% for charred MS.  Organic C associated with coarse particulate organic matter (POM) within the macroaggregates markedly increased by 65.1 and 41.2% for MS and charred MS, respectively.  Charred MS resulted in a noteworthy increment of 50.4% for organic C associated with heavy POM inside the occluded microaggregates, whereas charred MS and MS observably improved organic C associated with heavy POM inside the free microaggregates by 36.3 and 20.0%, respectively.  These results demonstrate that uncharred and charred maize straw amendments improve C sequestration by physically protecting more organic C in the macroaggregates and the occluded microaggregates.  Compared to the feedstock straw amendment, charred maize straw amendment is more advantageous to C sequestration.