This study aims to determine the differences in students' mathematics learning outcomes before and after online learning and their positive responses to this method's use. Data were quantitatively and non-randomly collected from Senior High School students in Madiun city, Indonesia, in May 2020, by using the documentation and questionnaire methods. There are two sources of data, namely mathematics learning outcome and student's response using an online questionnaire. Data were used to determine the effect of learning during the COVID-19 pandemic. Meanwhile, the questionnaire was used to determine their responses. The results showed that the average of mathematics learning outcomes before online learning is greater than the average after online learning and students' average positive response towards mathematics before online learning is greater than the average after online learning.