The genesis and differentiation into meristematic nodule of thin cell layer of (TCLs) petiole callus in modern tree peony ‘Golden era’ were studied.The results showed that there were four stages in the course of callus development,namely activation,division,formation and histodifferentiation.During activation,fascicular cambium cells were most notable,showing brown externally and bigger cells,clear and nearcenter nucleus internally.Meanwhile the starch metabolic enhanced.Cells kept division during the second stage division and led to the formation of callus in cambial area firstly and then in the region of cortex parenchyma cells.These callus tissues formed a compact cell cluster and the callus cells were characterized by dense cytoplasm,nucleus in center,small in volume and starch grains accumulated around the nucleus.However,callus proliferated at a slower rate during formation stage,only the peripheral callus cells which facing the medium kept division.At the last stage histodifferentiation,callus tissues differentiated into neoformed tracheary elements.Some of these tracheids organized as vascular centers and some specialized parenchyma cells could also act as centers.And then these centers were surrounded by loosely arranged,plastiddense cell layers.This way,meristematic nodule formed which was thought to be potential in mass production.The result here can be used as a basis for further improvement of organ regeneration system for largescale multiplication of tree peony.