Objective To observe the influence of longterm smoking on the expression of p53 and K-ras in rat lung tissues, and to study the relationship of smoking to the mutation of p53 and K-ras gene. Methods The model of Wistar rat smoking was built up. Seventytwo healthy male Wistar rats were divided into the experimental group and control group at random. The rats of the experimental group were compelled to smoke, and the rats of the control group were given the same condition as the experimental group was, without smoking. The rats of the experimental group were smoked for 6 months. At the end of each month, 6 rats were chosen from the two groups respectively, their lung tissues were sampled and immunohistochemistry was applied to observe the expression of p53 and K-ras in lung tissues. Finally, the mutation which might happen in the exon 5, 6, 78 of p53 and the exon 1 of K-ras was examined by PCRSSCP. Results The p53 protein was expressed in cell nucleus and K-ras in cytoplasm. The positive ratio of protein expression was increased with the extension of smoking time. The mutation of p53 was increased as the smoking time extended. But the effect of smoking time was not that significant on the mutation of K-ras.Conclusion Smoking can strengthen the expression of p53 and K-ras protein and can also result in gene mutation. As the time of smoking extended, those phenomenons were tending to rise. That provided the theoretical evidence which can be used to judge the lesion of lung tissues caused by smoking and help the early diagnosis of smokingrelated lung carcinomas. It