We develop a molecular dynamics simulation to evaluate the full Lyapunov spectrum for two-dimensional fluids composed of flexible diatomic molecules. Center-of-mass coordinates and angular variables are used for the configurational space variables. We obtain the Lyapunov spectra for 18 molecules for various force constants and densities with periodic boundary conditions. We discuss about different contribution of the phase space variables by calculating the projection of the certain subspace of the tangent space vector and compare our results with model uses the rigid diatomic molecules. The largest Lyapunov exponent and KS entropy trends of rigid models and flexible ones are nearly the same. But full Lyapunov spectra and tangent vector projection graphs were different. From this we can know the contribution of vibrational motion to unstability of the system.