Introduction: Many patients with cancer actively look for clinical trials online or in other places, hoping to find more options for treatment. With 2.5 billion smartphone users in 2019, an over 36 percent of the world's population, evidence suggested that mobile App is the most effective outreach channel among all the advertisement channels such as TV, radio, emails, social media and websites. Mobile Apps can immediately bring information on new studies to physician’s fingertip to refer patients. An App not only can check potential participants’ eligibility, inform patients’ decisions to sign up for the study, but also can track respondents’ characteristics and follow up with non-respondents. It could also allow patients to schedule appointments and medication reminders. Furthermore, because information is readily accessible from a smartphone, the recruitment process becomes more transparent and efficient. Our work: Here, we present our design of a mobile App to select and recruit subjects with relapsed and refractory multiple myeloma to participate in a Phase II clinical trial (KarMMa) for efficacy and safety study of the CAR T-cell therapy bb2121. Our App included comprehensive information on the disease condition, treatment options, study descriptions, eligibility questionnaire, visit schedules, site information, informed consent, and other supporting materials. Our App explained the science of CAR T-cell therapy in a language understandable by the general public. It also used graphics and short videos to enhance patients’ understanding. Ultimately, patients can make decisions independently with adequate information provided. Conclusions: Information, comprehension, voluntarily decision making, interactivity, as well as privacy protection are critical elements of a mobile App design to recruit subjects for clinical trials. A follow-up study to compare the recruitment results between the traditional approach and mobile App will be an exciting topic.