Reaction of thiosemicarbazide with Pyrazole carboxaldehyde derivatives to give the corresponding thiosemicarbazone which characterized by FTIR and 1HNMR, the product was investigated as corrosion inhibitors for carbon steel in HCl by chemical and electrochemical method. It has been observed that corrosion rate decreases and inhibition efficiencies increases with increasing in inhibitor concentration and temperature. The apparent activation energy (Ea) and other thermodynamic parameters have also been computed. The electrochemical data indicated the basicmodification of steel surface as a result in a decrease in the corrosion rate. The polarization data indicated that the used compound act as mixed type inhibitors. The slopes of the cathodic and anodic Tafel lines (âc, âa) are approximately constant and independent of the inhibitor concentrations. The inhibition occurs through adsorption of the inhibitor molecules on the metal surface without modifying the mechanism of corrosion process. SEM studies revels the formation of passive filmon themetal surface. Results obtained frompolarization, open circut and weight loss measurements are in good agreement with each other.