The ultrastructural changes during the development of sieve element (SE)/companion cell (CC) complex (SE/CC complex) in Populus deltoides were investigated using electronic microscopy. The result showed that SE and CC come from the same secondary phloem mother cell. The development of sieve element went through three stages, i.e. immature, mature, and degenerate stage. In immature stage, the sieve element presented radial expansion, cell wall thickening, and production of sieve plastid and P-protein. The cytoplasma selectivly autolysed in mature stage. At the same time, karyoplasm was dispersed, and nuclear envelope was not clear, or karyoplasm was disorgnized, and nuclear envelope was clear. The karyopalsm finally turned into P-protein. In degenerate stage, the cytoplasm completely disappeared, and the function was lost. There was lots of organelle, such as ribsomes, mitochondria in companion cell during the mature stage of SE. Also the plasmodesmata appeared in cell wall between SE and CC. With decline of function in SE, the CC presented on a typical characteristic of programmed cell death. The endoplasm reticulum cisterna was enlarged, and so did perinuclear space. Synchronously, the chromatin condensed and aggregated towards the nuclear envelope. At last, the CC was disappeared completely. In conclusion, the development of secondary phloem SE underwent denucleation firstly. And the cytoplasm degenerated gradually after carrying out some physiological function. Simultaneously, the nuclear and cytoplasm of CC degenerated. We presume that SE and CC took the same programmed cell death procedure an intact function complex.