Using the multi-phase field model proposed by Kim， Kim， Suzuki and Ode (KKSO)， numerical studies have been carried out to investigate the effect of lamellar thickness on the lamellar eutectic growth in three dimensions. Simulation results showed that the lamellar thickness has significant effects on the eutectic morphology evolution with a given lamellar spacing. When the lamellar thickness is comparatively small， the eutectic lamellar growth is similarly to the two-dimensional case. Increasing the lamellar thickness， the effect of lamellar thickness becomes stronger， as a result， the lamellar structure distorts vertically， and oscillations on both the direction of the width and the direction of the thickness occur alternatively after a further increase of the lamellar thickness. When the lamellar thickness is bigger than the lamellar width， a stronger oscillation in the thickness direction than that in the width direction occurs， and eventually the oscillation in the width direction is suppressed but only an approximate 1λ oscillation in the thickness direction can be found. Moreover， simulation results also showed that the change of undercooling is consistent with the morphology transition.