The present work aims at studying the physical mechanisms of nucleation site interaction in pool boiling. A series of experiments have been carried out to investigate the average bubble departure frequency (f) over bar (d) of two artificial cavities on the thin silicon surface for different dimensionless cavity spacing S/(D) over bar (b), that is the ratio of the cavity spacing to the average bubble departure diameter. Based on the comprehensive observation and analysis, three significant effect factors of nucleation site interaction are found out: A hydrodynamic interaction between bubbles; B thermal interaction between nucleation sites; C horizontal and declining bubble coalescences. The intensity, competition and dominance relations of these three factors determine four different interaction, regions: 'I' region (S/(D) over bar (b) > 3), 'H' region (2 < S/(D) over bar (b) less than or equal to 3), 'H + T' region (1.5 < S/(D) over bar (b) less than or equal to 2) and 'H + T + C' region (S/(D) over bar (b) less than or equal to 1.5). In the 'I' region, the bubble departure frequency is similar to that in the single cavity condition as a result of the negligible effect of all the three factors. For the 'H' region, the stronger influence of factor A results in a higher bubble departure frequency. Within the 'H + T' region, the co-existence and competition relation occurs between factors A and B. The dominance of factor B causes the lower bubble departure frequency. As for the 'H + T + C' region, although all the three factors are strong and compete simultaneously, the promotive effect becomes dominant finally and the bubble departure frequency increases again. Through the present study, some essential aspects of the nucleation site interaction in pool boiling have been revealed, which is significant for better understanding the boiling mechanisms.