Aliphatic poly(monothiocarbonate)s synthesized by alternating copolymerization of carbonyl sulfide (COS) and epoxides are a new type of crystalline polymers. In this work, the isothermal crystallization behavior and spherulite morphology of poly(trimethylene monothiocarbonate) (PTMMTC) and poly(ethylene monothiocarbonate) (PEMTC) are studied. PTMMTC tends to form banded spherulites, especially at low crystallization temperatures, while PEMTC forms non-banded spherulites. The overall crystallization rate and spherulitic growth rate of PTMMTC are larger than those of PEMTC at the same degree of supercooling. PTMMTC with longer alkane segment in the main chain has a higher equilibrium melting temperature (T-m(0)) than PEMTC. Both polymers exhibit double melting peaks, but the formation mechanisms are different for PTMMTC and PEMTC. Small-angle X-ray scattering (SAXS) result reveals that more amorphous phases are located in interlamellae of PTMMTC, leading to a larger long period (L) and a thicker amorphous layers (l(a)), which can account for the stronger tendency of PTMMTC to form banded spherulites. The melting enthalpies of PTMMTC and PETMC crystals with 100% crystallinity (Delta H-m(0)) are obtained by extrapolating the melting enthalpies of the samples with different crystallinities. The free energy of the folding surface (sigma(e)) of PTMMTC is larger than that of PETMC, indicating that the PTMMTC chain is stiffer. The higher T-m(0) and larger Delta H-m(0) and sigma(e)of PTMMTC can be attributed to its stronger interchain interaction.