Preparing students with appropriate abilities for taking some of on-line tasks is necessary. It will help students to build self-confidence and increase the probability to reach the success of learning. The purpose of this study aimed at equipping high school students with the skills of evaluating evidence and formulating evidence for taking an on-line task, in which students had to make a decision on choosing a location to build a reservoir. The developed instructional activity was a 4-hour unit, which provided a socioscientific context for students to understand the concept about evidence and discuss the reliability and validity of evidence to support if the global warming has been accelerating. The participant consisted of one earth science teacher and forty students. Two questionnaires and individual interviews were used to collect data. The results showed that the students had significant improvement in evaluating evidence, formulating evidence and justifying their arguments. Their understandings of criteria used for evaluating evidence became more clearly after the teaching. Most students reported that the instruction was beneficial for them to complete the decision-making task.