Resistive switching in transition metal oxides has attracted great attention due to the potential for Resistance Random Access Memories (RRAM) with high speed and density. Even though a great number of researches on resistive switching behavior of oxides have been conducted, it is still not fully understood the detail mechanisms such as electroforming and subsequent resistive switching in different materials, and such understanding is important to develop the new generation non-volatile memory materials or devices. TiO2, as a potential memristive material, has been largely studied and reported in the literature and it is an ideal candidate to study the mechanism of the resistive switching. On the other hand, TiO2 is an electronic-ionic semiconductor and it is reported that the formation and ordering of oxygen vacancies (ionic species) in the TiO2 thin film were resulted from the electrochemical reactions, which might lead to the phase change and finally the resistance switching. However, there are so far few studies to directly provide experimental basis for this ionic/electrochemical effects in the resistive switching processes of TiO2. In this study, the resistive switching behaviour of TiO2 thin film are studied by using Electrochemical Strain Microscopy (ESM) and conductive Atomic Force Microscopy (C-AFM) techniques. ESM is a new type of scanning probe microscopy technique, and it is an effective tool to study the local electrochemical phenomena such as surface deformation due to the electrochemical reactions, the ionic mobility and distribution, and the electrochemical activity. In this work, the TiO2 samples with various thicknesses were prepared on Pt/TiO2/Si substrates by pulsed laser deposition technique with different oxygen partial pressures. By combining ESM and C-AFM measurements, the ionic/electrochemical effects in the resistive switching processes of TiO2 are studied. The results have shown the direct correlations between the ionic/electrochemical processes and different stages of the resistive switching, including the electroforming, set, or reset. In addition, the possible mechanisms for the resistive switching in TiO2 are studied and discussed.