Abstract: 【Objectives】Plant tissue is widely used for the diagnose of plant nutrient status. The second functional leaf from the top of rice is easy sampled, with negligible injury to rice growth, and will not absciss during the whole growth stages.The feasibility of using the potassium content in the 2nd functional leaf as rice nutrition diagnosis method was studied. 【Methods】 A field experiment was conducted with different K2O application rates of 0, 60, 120, 180, 240, 300 and 360 kg/ha.The potassium contents in the 2nd functional leaf were tested at different growth periods and the grain yields were weighed.Regression analysis was performed to evaluate the relationship between the potassium contents in the 2nd functional leaf and the grain yields and K application rates.【Results】 The grain yields are significantly different under different K application rates. A quadratic plus plateau model well descriped the significant relationship between K application rates and the grain yield,satisfies the requirement of potassium status diagnosis. The potassium contents of the 2nd functional leaves was increased significantly with the increase of the K application rates. The K contants of the 2nd functional leaves was in range of 0.85%-2.72% from the beginning of the tillering stage to the full heading stage and reach the highest values at the jointing stage. The correlation analysis shows that the potassium content of the 2nd functional leaveswere significantly correlated with the grain yield and potassium application rates (P0.05). The yields are classified as five ranks, i.e., 85%, 85%-90%, 90%-95%, 95%-100% and 100% of the maximum yield. The corresponding K contents of the 2nd functional leaves were calculated according to the regression equation.When the 95% of optimal yield was set as the critical yield, the calculated K contents were 1.34%, 1.58%, 1.98%, 2.09%, 1.33% and 1.27% at initial tillering, active tillering, effective tillering, jointing, booting and full heading stages, respectively.【Conclusions】 The potassium content of the 2nd functional leaves are significantly related to the K2O application rates and yields, so, is suitable tobe used as the potassium status diagnosis index at the different rice development periods. If the 95% of optimum yield is set as the critical yield level, rice would be found potassium deficiency when the 2nd functional leaf contents are below 1.34%, 1.58%, 1.98%, 2.09%, 1.33% and 1.27% at the tillering stage, active tillering stage, effective tillering critical stage, jointing stage, booting stage and full heading stage, respectively.