To discuss the calculation methods under different sample size, used for diagnostic trials. The purpose of the diagnostic trial was to demonstrate the sensitivity and specificity of the new method. Equations and results were directly compared. Monte Carlo random simulation was used to validate the results. Sample size obtained from the sampling method was always smaller than from the target value method. Results from simulation showed that the target value method could offer more and larger power. The two sample size determination method showed essential differences of the results, suggesting that the investigator should choose appropriate method in accordance with the study design. If the hypothesis of study was to demonstrate the new diagnostic method which could meet the clinical requirements, only if the target value method provides enough statistical power.