Narrow-bandgap PbSnSe has received much attention as a promising alternative material for mid- and long-wavelength high performance of infrared detection at relatively high operating temperatures owing to the weak composition dependence of its bandgap, which can intrinsically result in better uniformity. Additionally, it possesses a high dielectric constant that is anticipated to be much more tolerant to defects. In addition, its growth by molecular beam epitaxy (MBE) can be easily accomplished in comparison with HgCdTe and many III–V quantum well and superlattice materials. However, overcoming the high lattice and thermal mismatches between PbSnSe and CdTe/Si substrates and improving the crystalline quality of PbSnSe grown on CdTe/Si substrates are challenges that require further study. Additionally, interdiffusion between CdTe and PbSnSe can take place and lead to nonuniform distributions of elements in PbSnSe. Epitaxial crystal PbSnSe alloy films were grown by MBE and were investigated by scanning and high-resolution transmission electron microscopy (STEM/HRTEM). Etch pit density (EPD) measurements were done to determine the density of threading defects in the films. EPD measurements on PbSnSe surfaces gave values in the mid-106 cm−2 range. The dislocations exposed as etch pits were found to accumulate and form small-angle grain boundaries lined up along the (011¯¯¯) direction, which is the intersection line between (100) and (211) growth planes.