To accelerate drug discovery and development, improve R&D efficiency and facilitate scientific decision making, quantitative systems pharmacology (QSP) is increasingly being utilized in the pharmaceutical and biotech industry. QSP approach has emerged as a central technology in innovative framework based on "learn and confirm" cycle, which is currently called as model-informed drug discovery and development (MID3). QSP approach is supporting the rational decision in drug discovery and development from exploratory research to late-stage clinical development, and enables selection/validation of novel target molecules, identification of mechanism-of-action of pharmacological/toxicological compounds, prioritization of pharmaceutical candidates, facilitating translational research, optimization of clinical trials, and others. In this review paper, I explained summary and effectiveness of QSP approach by showing two our case-studies related to identification of mechanism-of-action of compounds. These are 1: differentiation of thrombin inhibitors and 2: endocrine disrupting compounds, they have been good practices in my experiences of applying QSP at pharmaceutical company.