Thermal effects of different applicators for energy deposition in tissue undergoing laser interstitial thermotherapy (LITT) are investigated. The aim is to predict temperature distribution (T), dimensions and shape of thermal lesion produced by the laser light absorption within the tissue, in order to achieve an optimal design of the applicator for LITT. A numerical model, based on Monte Carlo method, was implemented to predict the distribution of laser light within the tissue, and the Bio Heat Equation was used to simulate T. Four geometries of optical applicators with different emitting surfaces were considered: bare fiber, cylindrical, zebra and a hybrid geometry. Effects on liver tissue undergoing LITT were evaluated in terms of T and coagulation volumes. Simulations were performed with laser power of 3 W and 5 W and energy of 1650 J. Results show that bare fiber causes an irregular coagulation shape; zebra and hybrid applicators 3 cm-long obtain an elliptical lesion, with lowest maximum T (Tmax) on their surface (about 350 K); cylindrical applicators with length of 1 cm or 1.5 cm produce spherical lesions, with Tmax up to 398 K. Results suggest that the selection of the applicator based on the geometry of the lesion could improve LITT outcome. Furthermore, an appropriate geometry can preserve applicator integrity avoiding excessive temperature increase on its surface.