Low-temperature solution processable TiO2 nano-sol for electron transporting layer (ETL) of flexible perovskite solar cells is prepared by synthesizing TiO2 nano-sols by peptization in acidic aqueous solution and re-dispersing the TiO2 nanoparticles in H2O, ethanol (EtOH), dimethylsulfoxied (DMSO), and N,N-dimethylformamide (DMF). By considering dielectric constant, zeta potential, and surface tension of solvent media, we found that the DMF is the best to form uniform TiO2 ETL by spin-coating among them because it has low surface tension and high zeta potential. Accordingly, we could demonstrate 18.2% power conversion efficiency (PCE) for rigid glass substrate based perovskite solar cells and 15.8% PCE for flexible polymer substrate based devices by low-temperature solution process.