As potential materials for optic biosensors, the surface and optical properties of bio-functionalized polydiacetylene Langmuir-Blodgett films are interesting and important. p-10,12-Pentacosadiyne-1-N-phenylamide alpha-D-mannopyranoside (MPDA) was synthesized from 10,12-pentacosadiynoic acid (PDA). Pressure-area isotherms of pure and mixed MPDA/PDA monolayers were obtained. The analysis of the collapse pressures and the excess free energies of the isotherms shows that MPDA/PDA monolayers have good miscibility.