Experiments were conducted to study the morphological changes and functions of organelle during oogenesis in Brachymystax lenok by histological method and transmission electron microscopy.The mitochondria were found from oogonium stage to mature oocyte stage.They were round in the first phase,and became rod-shaped,curved or elongated with their proliferation at phase Ⅱ.The cristae and matrix were well formed and their number increased progressively,leading to the formation of mitochondria clusters.At the end of phase Ⅲ,mitochondria turned into round shape and divided by bipartition or gemmation,and some mitochondria lost their cristae and became vesicles,and their electrondensity also decreased.During this stage,mitochondria might play a relevant role in the formation of vesicles,preparing for accumulation of pre-vitelline elements.A small number of Golgi body and endoplasmic reticulum appeared near the nucleus in the early stage of phaseⅡ,and their structure was simple.In the latter phases,with their development,Golgi body and endoplasmic reticulum played a great role in synthesis and processing of vitelline materials.Most endoplasmic reticulum were arched in shape,and some were round or calathiform in shape.With their proliferation,endoplasmic reticulum performed their synthesis function.At the early stage,Golgi bodies were also arched in shape,and with the development of oocytes,a few scattered Golgi bodies gathered to form developed Golgi complex accompanied by numerous vesicles and annulate lamellae (AL).AL were curve in shape and their structure was similar to nuclear envelope.It is presumed that the annulate lamellae are derived from nuclear envelope,and its main function may be the storage for the membrane.