Using GaAs-on-InP heteroepitaxial technology, a long wavelength transmitter OEIC was fabricated, in which a GaInAsP/InP laser was monolithically integrated with a GaAs MESFET. A direct modulation of 5Gb/s NRZ is demonstrated on this OEIC. This is the highest bit rate for GaAs-on-InP OEICs and is comparable to the best result ever reported on OEICs in general.