Gauging the effectiveness of specific teaching strategies remains a major topic of interest in science education. Inquiry teaching among others has been supported by extensive research and recommended by the National Science Education Standards. However, most of the empirical evidence in support was collected in research settings rather than in normal school environments. Propensity score analysis was performed within the marginal mean weighting through stratification (MMW-S) approach to examine the effects of the level of openness of inquiry teaching on student science achievement and attitudes with the Programme for International Student Assessment (PISA) 2006 data. Weighting subjects on MMW-S weight successfully balanced all treatment groups on all selected covariates. Significant effects were identified on both cognitive and attitudinal outcomes. For student science achievement, the highest score was achieved at Level 2 inquiry teaching, that is, students conduct activities and draw conclusions from data. For student science attitudes, higher level of inquiry teaching resulted in higher scores. The said conclusions were generally held in most PISA 2006 participating countries when the analysis was performed in each country separately.