Abstract: Spore development in Phaeoceros carolinianus (Michx.) Prosk. was studied using transmission electron microscopy. Observations showed that:(1) The mature spore wall of P. carolinianus moss was composed of six layers, including (from inner to outer) thin inner layer of intine, thick and loose external layer of intine, homogeneous exine layer, thin layer of perine 1, loose structured layer of perine 2, and discontinuous layer of perine 3. The spiny and papillary ornamentations on the spore surface were determined by the exine layer. Spore wall development was incompletely centripetal. Spore development was similar to that of mosses in structure and sequence of spore wall development. (2) Primexine occurred in spore development of P. carolinianus, participated in the formation of the exine. Primexine formation type was the same type as liverworts, and the formation material of the exine was exogenous. The endoplasmic reticulum and dictyosome participated in the formation of the intine. (3) The perine layer of P. carolinianus could be divided into three asymmetric thickening layers, with characteristics similar to that of some ferns. In consideration of complex stratification (three layers), we speculate that the perine of P. carolinianus belongs to a more evolutionary type in bryophytes.