Cancer is among the world's most deadly inflictions, and early diagnosis is critical. Aptamers have shown utility as cancer probes since they can be screened rapidly in vitro against cancer tissues using systematic evolution of ligands by exponential enrichment (SELEX) process. However, bench-top SELEX procedures are relatively labor-intensive and time-consuming; ideally, they could instead be carried out on microfluidic devices, yet this requires optimization of buffer and reaction conditions. Herein an integrated microfluidic system (IMS) was established to automatically carry out the optimization of aptamer selection. A "formulation chip" was developed that could mix salt solutions at differing final concentrations, and the resulting optimal binding buffer was transferred to another "optimization-SELEX chip" for the following tissue-SELEX. Two aptamers were successfully screened; one of which, H-45, exhibited high specificity and affinity towards ovarian cancer tissue samples, suggesting that this IMS might be a promising device for screening of cancer associated aptamers for cancer diagnosis.