Lattice thermal conductivity is a function of the velocity and mean free path of phonons, which can be strongly correlated to phase transition of solids. Herein we describe our recent progress of studies of the phase transition in the molecular crystals by means of thermal conductivity measurements. Thermal conductivity κ and specific heat C were measured by a home-made apparatus, and product of mean free path of phonon l and phonon velocity v was estimated by κ = 1/3Cvl. In most of the case, minima of κ and vl values were found near to the transition temperatures. Below the transition temperature,κ and vl value followed the T−1 functions below the transitions temperature, which meant that the phonon transportation was ballistic. Above transition temperature, temperature coefficient of κ value was positive while vl value was almost constant, as l value was minimized and phonon transportation became diffusive. Such the behavior was observed for 2D organic-inorganic perovskite, the one-dimensional hydrogen bonding ferroelectrics, the one-dimensional semiconductors of TCNQ0.5- salts, and the Fe(III) spin crossover complexes. Our home-made apparatus for thermal conductivity measurement was also described in detail.