Rotational speed of optical drives and servo speed of optical pick-up actuators should be higher to meet high performance for optical drives. One of the methods to achieve such a high performance product is to increase an input power for the coil, but a heat problem generated by the coil will become serious. Two kinds of new ferrofluids have been designed to transfer a heat from the coil of optical pick-up actuator. Their physical properties have been properly designed to be functioned at a wide operating temp. and various aspects of their reliability including resistance for deterioration to high temp. and humidity, etc., have been evaluated. Also, they have been applied to optical pick-up actuators and the properties of ferrofluids such as heat transfer and shock resistance have been examined. The life tests showed that the new ferrofluids had at least two times longer life and a smaller viscosity change than CF-3A, which has been used in commercially available optical pick-up actuators. The new ferrofluids could tolerate the shock of 500 G without a ferrofluid splash, the coil temp. with the ferrofluid was about 10 degrees C or more lower than the one without it at the same input power.