Abstract: In order to explore the cytological mechanism of cold resistance in alfalfa crown buds during winter, a study of ultrastructural changes in young leaf cells of crown buds in the alfalfa variety 'WL168' was conducted by means of TEM. The results showed that: 1) a series of active changes in cell ultrastructure adapted to cold temperature take place during overwintering. The major changes included plasmalemma invaginations, the large central vacuole segmentation into multiple small vacuoles, chromatin condensation, plastid becoming crescent or horseshoe-shaped, starch grains depletion or even disappearance and plasmolysis. The capacity for cold resistance in buds was improved gradually by these adaptive adjustments of cell ultrastructure. 2) Two kinds of crown buds were found in winter: white bud and brown bud. These two kinds have different cellular growth states. The white bud cells had a slow response to cooling before winter and did not fully form specialized structures for winter hardiness until soil freezing, but they regrew quickly after cold resistance release in early spring. The cold resistance development of white buds during overwintering went through 4 phases: enhanced phase (late Oct-mid-Dec), maintenance phase (late Dec-mid-Jan), decline phase (late Jan-late Feb) and termination phase (early Mar-mid-Mar). In comparison, brown bud cells responded earlier to low temperatures before winter but more slowly to the warming of temperatures in spring. By mid-March, the cellular structure of brown buds still maintained their cold tolerance. The coexistence of brown and white buds during winter, whose cell ultrastructure changes asynchronously in response to low temperature, could be an adaptive strategy of alfalfa to cold stress.