Single crystals of poly(beta -propiolactone) (PPL) with different molecular weights (M-w = 70000 and M-w = 2300) were grown from four kinds of solvents under isothermal crystallization condition. The morphologies and crystal structures of PPL single crystals were investigated by means of transmission electron microscopy and atomic force microscopy. The single crystals of high-molecular-weight PPL (HMW-PPL) grown from cyclohexanone appeared elongated with dimensions of around 0.2-0.8 mum along the short and 5-10 mum along the long axes. Single crystals of low-molecular-weight PPL grown from cyclohexanone showed three to five elliptical-shaped lamellae, from central nucleus like petals. The long axes of both single crystals corresponded to the crystallographic b axis. The reciprocal lattice parameters: a* = 2.045 nm(-1), b* = 1.420 nm(-1) and gamma* = 90 degrees could be determined from electron diffractograms. Decoration of the crystals with polyethylene suqqested that the single crystals of HMW-PPL have regular chain-folding along their long axis in the [010] direction with consecutive folds in the [110] and [1 (1) over bar0] directions. Accordingly, it is deduced that HMW-PPL has the anti-parallel chain-packing structure.