This study was aimed at investigating the effect of the portfolio in Physics teaching on scientific attitude by controlling prior knowledge. This study was conducted at public senior high schools in Singaraja by using the experimental method with single factor independent group design with the use of covariable. This study involved 152 Class X MIPA students of senior high schools as a sample selected by multistage random sampling technique. Portfolio assessment was made by integrating it with Physics teaching. The implementation of portfolio assessment involved 4 basic elements: the presence of the student\u27s work folder, clear assessment criteria, self-assessment, and teacher and student conference. The data that were needed in this study were students\u27 scientific attitude as the independent variable and prior knowledge as covariate variable
