Objective To explore the feasibility of the research method which will fill one third of the root canal and then do the post space preparation immediately through apical micro leakage. Methods 56 extracted premolars, each with single root canal were randomly divided into seven groups: Group A1 and group A2 are the immediate post space preparation group; group B1 and group B2 are the delayed post space preparation group; group C1 and group C2 are the intact group; group D is a control group. Groups A1, B1 and C1 were restored by the root canal filling with conventional and cold lateral compression while one third of the root canal filling group A2, B2 and C2. Group D received no treatment. Ink dye method was used to measure the apical leakage in stereo microscope. Results The mean length of dye penetration for group A1, A2, B1, B2, C1, C2 were （1.92± 0.23）, （1.89±0.43）, （2.37±0.40）, （2.33±0.16）, （1.68±0.20）, （1.70±0.30）mm respectively. Group D demonstrated maximum dye penetration. There were statistically significant difference between groups A1, A2, C1 and C2（P< 0.05）. There was not statistically significant difference between group B1 and group B2（P<0.05）, while there were statistically significant difference between group A1, A2, C1 and C2（P>0.05）. Conclusion There is some clinical and practical value in one third of the root canal filling the immediate post space preparation.
