In this report, two different TiO2 solutions with and without HCl were, firstly, prepared. Then, totally twelve number of thin film samples were obtained using these solutions for each of these two different group films by ultrasonic spray pyrolysis (USP). Of these twelve samples, each of the four was created at three different spray times (25, 50, 75 min), respectively, and they were annealed at 500 C. Structural, morphological, and electro-optical properties of TiO2 thin films were performed by X-ray diffractometer (XRD), scanning electron microscope (SEM), atomic force microscope (AFM), and ultraviolet-visible spectrophotometer (UV-VIS). It was seen that the HCl addition improves the crystallinity of the thin film samples dramatically even though at low deposition temperatures. HCl addition causes densification on the surface of the films and these films also exhibited the best morphological and structural properties compared to thin films without HCl. Also, the bandgap values of all thin films prepared with and without HCl decreases from 3.40 to 3.21 eV and 3.29 to 3.15 eV, respectively. Increase in the thickness of films by the addition of HCl plays a vital role on the morphological, structural, and electro-optical properties of the samples.