This paper addresses the lack of large-scale open-source datasets for scientific reasoning by presenting the TextbookReasoning dataset, which contains 650,000 inference questions extracted from college-level science textbooks, and the MegaScience dataset, which contains 1.25 million instances integrated from various open-source datasets. MegaScience was developed by systematically identifying optimal subsets through ablation studies of various data selection methodologies. Furthermore, a comprehensive evaluation system encompassing 15 benchmarks ensures accurate evaluation metrics. Experimental results demonstrate that the proposed dataset outperforms existing open-source scientific datasets in terms of performance and training efficiency. Baseline models trained on MegaScience—Llama3.1, Qwen2.5, and Qwen3—significantly outperform their corresponding official instruction models on average. This paper contributes to the advancement of scientific reasoning research by disclosing the data cleaning pipeline, evaluation system, dataset, and seven trained models.