Digital Image Analysis is a new DNA content measuring method in cell nucleus. During the senile process of Tetrahymena pyriformis,change of DNA content in nucleus could be measured by this method. According to law of Beer-Larmbert,cell nucleus in different growth period showed change of nuclear DNA content using level of nuclear integral optical density.The method possess quick measuring speed, well repetition, simple operation and good results.The results showed:when Tetrahymena pyriformis began the logarithmic growth phase, the nuclear DNA content reached peak gradually. When cell ageing gradually, the times of cell division as well as DNA content would be gradually decreased.