Abstract: 【Objectives】 Structure of controlled-release film of polymer, prepared by physical covering methods, directly determines the mass transfer of nutrients. The main channel for the release of nutrients is the micropores and cracks in the films. Bubble point method is widely employed for measuring the diameter and their distribution of available pores in the films. In this study, the method was modified for the determination of the maximum aperture in the polyolefin films, which is used for coating of fertilizer, and the parameters for establishing a standard method were testified.【Methods】 On the base of the bubble point method, a device for the determination of maximum pore diameter of the film were installed, and the location of maximum aperture in the film was determined. The morphology and structural characteristics of pores on the film was observed by scanning electron microscope. The optimum parameters for the modified method were determined by the comparative analysis of different infiltration agents, temperatures and time. The correlation between the releasing period and the maximum pore was studied using polyolefin coated fertilizer with releasing periods of 1, 3, 5 and 6 months.【Results】 1)After the four types of fertilizers were soaked into water, the granules with urea crystallization of white point in the film surface are gradually increased with the elongation of the soaking. The white points could be thought as the releasing channel of fertilizer core nutrients. The white points of granule are set as the position for the determination of the maximum aperture film. At the 10th day of soaking, the bubble point could be detected in more than 80% of the granules. The characteristics of these pores or holes are matched with those observed by scanning electron microscope. 2)The maximum pores of the film are determined directly by the pressure bubble method with self-made apparatus at temperature of 25℃, Q-16 is selected as wetting agents and soaked for 5 min. 3)The average pore sizes of the coated fertilizer with releasing periods of 46, 105, 160 and 198 d are 1.93, 0.58, 0.45 and 0.41 m, respectively. The maximum pore sizes are increased with the shortening of releasing period and increasing of nutrients. Therefore, a close relationship exists between the maximum pore size and the release performance. 【Conclusions】 The bubble point method can be used as a standard method for the determination of maximum pore diameter, there is relationship between the determination of maximum aperture and the release time, it plays an important role in further understanding the controlled release performance and coated controlled release mechanism of fertilizer.