In order to gain possible hints for planet formation from the current data of known extra-solar planets, the period-ratios and mass-ratios of adjacent planet pairs in multi-planet systems are determined. A moderate period-ratio-mass-ratio correlation is found to have a correlation coefficient r=0.5779 with 99% confidence interval (0.464, 0.672). In contrast, for non-adjacent planet pairs, the correlation coefficient is r=0.2820 with 99% confidence interval (0.133, 0.419). Our results reveal the imprint of planet-planet interactions of the adjacent planet pairs in a certain fraction of the multi-planet systems during the stage of planet formation.
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