Abstract Ultrastripped supernovae (USSNe) with a relatively low ejecta mass of ∼0.1 M ⊙ (e.g., iPTF 14gqr and SN 2019dge) are considered to originate from ultrastripped carbon–oxygen cores in close binary systems and are likely to be progenitors of binary neutron stars. Here we conduct the explosion simulations of ultrastripped progenitors with various masses (1.45 M ⊙ ≤ M CO ≤ 2.0 M ⊙) based on results of neutrino-radiation hydrodynamics simulations, and consistently calculate the nucleosynthesis and the supernova light curves. We find that a USSN from a more massive progenitor has a larger ejecta mass but a smaller 56Ni mass mainly due to the fallback that leads to the light curve being dimmer and slower. By comparing the synthetic light curves with the observed ones, we show that SN 2019dge can be solely powered by 56Ni synthesized during the explosion of a progenitor with M CO ≲ 1.6 M ⊙ while iPTF 14gqr cannot be explained by the 56Ni-powered model; ∼0.05 M ⊙ of 56Ni inferred from the light-curve fitting is argued to be difficult to synthesize for ultrastripped progenitors. We consider fallback accretion onto and rotation-powered relativistic wind from the newborn neutron star (NS) as alternative energy sources and show that iPTF 14gqr could be powered by a newborn NS with a magnetic field of B p ∼ 1015 G and an initial rotation period of P i ∼ 0.1 s.