Flip-chip bonding of various optical components (laser diode, photodiode, grating, etc) on a silicon substrate by passive optical alignment, which eliminates complicated optical axis precise alignment, has opened up many new possibilities in constructing highly functional, reliable, and low-cost optical micro-systems. Using this technique, several microsensors (< 2 × 3 mm 2 ), including a blood flow sensor that can monitor blood flow in human skin, and a high-resolution encoder that can measure displacements or revolution angles, have been developed. Moreover, for future optical micro-systems, a novel method of a low temperature flip-chip bonding using surface activated bonding process has been introduced.