Alignment structure and optical properties of Langmuir- Blodgett ( LB) films of urethane- substituted polydiacetylene ( poly-4BCMU) was investigated. During compression, the coil to rod transition in molecular conformation of poly- 4BCMU was induced and the polymer main chains were aligned perpendicular to the compressing direction. The dichroic ratio of absorption intensity of poly- 4BCMU monolayer film was about 6. LC molecules were sandwiched with two glass plates covered with the poly- 4BCMU monolayer films. LC molecules aligned spontaneously along the main chain direction of poly- 4BCMU in the parallel cell. On the other hand, they formed a twisted structure of 90 degrees in the crossed cell. The monolayer film of poly- 4BCMU had a high potential as rubbing- free alignment layer of LC molecules.