Abstract: Objective Because the sample size estimations currently used have considered inappropriately the influence of the population mean differences of two groups in equivalence clinical trials, the actual powers could not reach the objective level of power predetermined. This paper was to explore the sample size estimation of equivalence clinical trials which the primary variable follow normal distribution. Methods The formulae of sample size estimation were derived based on the statistical inference principle of equivalence clinical trials incorporated with the decomposition of β and non-central t distribution theory. According to the formula, the sample sizes were estimated under different parameter settings. Monte-Carlo simulations were performed to obtain the corresponding powers respectively. Results The achieved power of Monte-Carlo simulation could coincide with the pre-determined level of power well. It showed that the method possessed correctness to estimate the sample size. Conclusion The method of sample size estimation could be used commonly in equivalence clinical trials with quantitative normal distribution variable.