In this paper, using molecular dynamics simulations we report spontaneous curling behaviors of freestanding Janus monolayer S–Mo–Se (MoSeS) structures. Density functional theory calculations are performed to obtain the phonon dispersion and phonon spectra of the Janus monolayer MoSeS for analyzing its structural stability. The results show that the Janus monolayer MoSeS is structurally stable. Due to the lattice mismatch between MoS and MoSe domains, the Janus monolayer MoSeS at the freestanding state always spontaneously rolls up in a constant temperature and pressure system. The direction of curling is preferred along the armchair orientation. Specifically, as for the Janus monolayer MoSeS whose size is larger than ∼30 nm, it can spontaneously roll up into a nanotube structure. The underlying physical mechanisms of these phenomena are well uncovered by using classical Timoshenko plate theory and the minimum energy principle.