Emission from a single-mode 100 mW diode laser at 840 nm is used to create optical tweezers: the trappinglaser beam is introduced into a microscope and focused by the objective. The microscope also allows monitoring of the motion of the trapped particles. The optical tweezers were monitored with objectives having different numerical apertures between 0.65 and 1.3. The optical trapping of polystyrene spheres with a radius between 0.11 and 7.45 μm and of biological objects, the flagellated alga Tetraselmis, with typical dimensions of 8×8×13 μm3 were studied. The efficiency of the optical tweezers has been characterized through a parameter Q and compared with theoretical models.