Dendritic spines are postsynaptic structures that receive excitatory synaptic signals from presynaptic terminals in neurons. Because the morphology of spines has been considered to be a crucial factor for the efficiency of synaptic transmission, understanding the mechanisms regulating their morphology is important for neuroscience. Actin filaments and their regulatory proteins are known to actively maintain spine morphology; recent studies have also shown an essential role of microtubules (MTs). Live imaging of the plus-ends of MTs in mature neurons revealed that MTs stochastically enter spines and mediate accumulation of p140Cap, which regulates reorganization of actin filaments. However, the molecular mechanism by which MT dynamics is controlled has remained largely unknown. A cell polarity-regulating serine/threonine kinase, partitioning-defective 1 (PAR-1), phosphorylates classical MAPs and inhibits their binding to MTs. Because the interaction of MAPs with MTs can decrease MT dynamic instability, PAR-1 is supposed to activate MT dynamics through its MAP/MT affinity-regulating kinase (MARK) activity, although there is not yet any direct evidence for this. Here, we review recent findings on the localization of PAR-1b in the dendrites of mouse hippocampal neurons, and its novel function in the maintenance of mature spine morphology by regulating MT dynamics.