Some sources of Leonardo's studies are presented; these can be clearly recognized by comparing some Da Vinci's designs and drawings with those by other scientists, engineers and architects of the past. The examined period of time ranges from Ⅲ century B.C. up to Da Vinci's coevals. The main fields that were considered are the human anatomy, the mechanics (self-propelled carts, gears, weapons, lifting machines, hydraulic saws and robots), paddle ships and flying machines. These sources mainly consist in what survived from the Hellenistic knowledge and in the Islamicculture. Some of those examples show such a modernity that even the modern scientist can be surprised since the modern knowledge and devices was sometimes anticipated by their colleagues of previous time. The present study, hence, shows an important aspect of Da Vinci showing him as a meticulous researcher of the past.