The individual problem-solving (IPS) and collaborative problem-solving (CPS) have received a lot of attention, yet little research has been conducted to investigate whether CPS and IPS are equally effective in improving students' understanding of physics concepts, problem-solving abilities, and minimizing achievement gaps. Therefore, the present study developed two types of online electromagnetism problem solving programs with simulation--IPS and CPS--for 8th grade students over five class sessions. Students in the CPS group significantly outperformed those in the IPS group on their performance of physics problem solving test and online problem-solving solution, while IPS and CPS both affected their physics concept test performance to the same degree. The CPS group allocated more time to the online problem-solving solution, evidence-based reasoning, simulation and data reporting than the IPS group. Both CPS and IPS affected high-achievers' problem-solving performance to the same extent. Nonetheless, CPS was more effective in maximizing low-achievers' problem-solving performance and minimizing the discrepancy between high- and low-achievers than IPS, possibly because low-achievers in CPS group requested and received more support from high-achieving students. Regression analysis indicated that students' online problem-solving solution significantly predict their posttest performance in the physics concept test and physics problem-solving test.