This paper presents direct measurements of the aerodynamic forces on the wing of a free-flying, insect-like ornithopter that was modeled on a hawk moth (Manduca sexta). A micro differential pressure sensor was fabricated with micro electro mechanical systems (MEMS) technology and attached to the wing of the ornithopter. The sensor chip was less than 0.1% of the wing area. The mass of the sensor chip was 2.0 mg, which was less than 1% of the wing mass. Thus, the sensor was both small and light in comparison with the wing, resulting in a measurement system that had a minimal impact on the aerodynamics of the wing. With this sensor, the 'pressure coefficient' of the ornithopter wing was measured during both steady airflow and actual free flight. The maximum pressure coefficient observed for steady airflow conditions was 1.4 at an angle of attack of 30 degrees . In flapping flight, the coefficient was around 2.0 for angles of attack that ranged from 25 degrees to 40 degrees . Therefore, a larger aerodynamic force was generated during the downstroke in free flight compared to steady airflow conditions.