In this study dissimilar joining of St12 to Polycarbonate is accomplished by a Nd: YAG pulsed laser and examined by finite element (FE) model and analysed by statistical method. Several experiments are carried out to materialize a direct joint between St12 to Polycarbonate. To investigate thermal phenomena of the laser joining process a FE model is developed by Abaqus software. To approximate heat source distribution, a Cylindrical-Involution-Normal model is programmed in FORTRAN language. To find out the sensitivity of the FE model to the elements size, a number adjustment tests are used and the FE model is validated by experimental data. Effects of laser power (190−230 W) and laser scanning speed (3.6–7.6 mm/s) on average bond width (AW), delta bond width (DW), and maximum of temperature profile (MT) have been investigated via response surface methodology. Results reveal that power of laser is the determinant variable of average bond width and maximum temperature profile, however, scanning speed of laser is the most effective variable on delta bond width. An appropriate process window required to achieve a sound dissimilar joint (without any decomposition of the polymer) is finally suggested.