The influence of the flexible alkyl chain spacer on the molecular dynamics of glass transition in side-chain liquid-crystalline polymers (SCLCPs) and its dependence on the spacer length are considered in the framework of the Coupling model. Short spacer length may enable the mesogenic groups in the side chain to enhance the degree of cooperativity of the segmental dynamics responsible for glass transition compared with that of the corresponding parent main-chain polymer without the side chains. However, when the spacer length becomes long, not only motions of the mesogenic group and the main chain are decoupled but also the preponderance of flexible methylene chains in the vicinity of the polymer backbone will diminish the degree of cooperativity of the segmental dynamics compared with that of the corresponding parent main-chain polymer without the side chain. The degree of cooperativity is quantified in the coupling model for SCLCPs by the steepness of the plot of a characteristic relaxation time, tau*, of the segmental relaxation time spectrum against normalized reciprocal temperature, T-g/T, where T-g is operationally defined as the temperature at which tau*(T-g) is equal to an arbitrarily chosen but fixed long time. Dielectric and mechanical relaxation data of several families of SCLCPs published in the past 10 years are used to test this prediction. The results from the analysis of these data are in accord with the predicted trends.