This work demonstrates the feasibility of the generation of an RF direct-detection vector signal using optical in-phase/quadrature-phase (I/Q) upconversion. The advantage of the proposed transmitter is that no electrical mixer is needed to generate the RF signal. Therefore, I/Q data of RF signals are processed at baseband at the transmitter, which is independent of the carrier frequency of the generated RF signal. A 10 Gb=s 16 quadrature amplitude modulation signal is experimentally demonstrated. Following transmission over a 50 kmsingle-mode fiber, the power penalty is negligible. Moreover, I/Q imbalance of the proposed transmitter is studied and compensated by digital signal processing, which is both numerically and experimentally verified.