The stacking structure of Nb2CSe2, a newly synthesized layered metal carbo-selenide, was elucidated by scanning transmission electron microscopy. Nb2CSe2 features Se−Nb−C−Nb−Se quintuple atomic layers. These layers are stacked in Bernal mode. In this mode, Nb2CSe2 crystallizes in a trigonal symmetry (space group P
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Twin boundaries have been exploited to stabilize ultrafine grains and improve mechanical properties of nanomaterials. The production of the twin boundaries and nanotwins is however prohibitively challenging in carbide ceramics. Using a scanning transmission electron microscope as a unique platform for atomic-scale structure engineering, we demonstrate that twin platelets could be produced in carbides by engineering antisite defects. The antisite defects at metal sites in various layered ternary carbides are collectively and controllably generated, and the metal elements are homogenized by electron irradiation, which transforms a twin-like lamellae into nanotwin platelets. Accompanying chemical homogenization, α-Ti3AlC2 transforms to unconventional β-Ti3AlC2. The chemical homogeneity and the width of the twin platelets can be tuned by dose and energy of bombarding electrons. Chemically homogenized nanotwins can boost hardness by ~45%. Our results provide a new way to produce ultrathin (< 5 nm) nanotwin platelets in scientifically and technologically important carbide materials and showcase feasibility of defect engineering by an angstrom-sized electron probe.