A 1.7kW CO2 laser was used for the study of bead-on-plate welds in D36 ship steel. The effect of welding heat input and focal point position on weld geometrical features and microstructure was determined. It was found that weld penetration as well as the width of weld pool and heat-affected-zone (HAZ) increase with heat input. Microstructure and hardness in the weld pool, Partial fusion zone and HAZ are also influenced by heat input. As the heat input is increased, the associated cooling rate is decreased, resulting in the formation of softer microstructures. Simple analytical models describing heat flow during welding were used to calculate the above mentioned geometrical features. The agreement between the calculated and experimentally determined weld penetration, as well as weld pool width, is sufficiently good and depends on weld heat input. The optimum experimental conditions defined from the bendon-plate study were applied to Laser welding of butt joints in 4mm D36 steel.