Gelidium corneum films were prepared using cinnamaldehyde as a cross-linking agent and their physical properties were determined. Tensile strength (TS) value of the film containing 0.01% cinnamaldehyde was higher than the control by 8.31 MPa. However, increasing cinnamaldehyde from 0.01% to 0.1% significantly decreased TS from 9.54 MPa to 0.03 MPa, and no film was formed at 1% cinnamaldehyde. On the contrary, when cinnamaldehyde content was increased from 0.01% to 0.1%, % elongation was increased from 1.44% to 2.75%. Water vapor permeability (WVP) of the film containing 0% and 0.01% cinnamaldehyde were 1.64 ng m/m$^2$sPa and 1.42 ng m/m$^2$sPa, respectively. There was no significant difference in Hunter values among treatments. Scanning electron microscopy results revealed that both cinnamaldehyde and control films had similar surfaces. These results suggest that 1.5% Gelidium corneum treated with 0.01% cinnamaldehyde should be the most suitable condition for film formation.