Objective To investigate the electrocardiogram（ECG） characteristics and mechanism of basic cardiac intermittent bundle branch block（IBBB） caused by premature beats. Methods The data of 8 typical cases were analyzed and discussed. Results Premature beat could lead to basic heart beat IBBB

which could lead to the intermittent appearance of left/right bundle branch block（L/RBBB） and the transient disappearance of L/RBBB.Conclusions The pathogenesis of IBBB induced by premature beats is related to one or more of the following factors:（1）Frequency-dependent BBB（3-phase

4-phase

3+4-phase）;（2）Premature beats lead to "dyssynchrony" of the left and right bundle branches. The time of one bundle branch is advanced

and the time of the other bundle branch is delayed

so that the following basic beats can only be transmitted down the one bundle branch

resulting in the generation of IBBB;（3） The phenomenon of repeat: sometimes BBB once appeared

there is a persistent（endless）trend; This phenomenon can be terminated by premature contractions;（4）Ashman phenomenon.