The performance of space time trellis codes (STTCs) can be further improved by utilizing the partial channel information and combining some precoding schemes at the transmitter. This paper presents a STTC scheme - hybrid STTC (HSTTC). For this scheme, the STTC signals are first linearly transformed and then transmitted through the single selected antenna. The upper bound on pairwise error probability (PEP) for HSTTC is derived. It is shown that by a proper code construction HSTTC can achieve not only a full spatial diversity order, but also a time diversity order, as well as a significant coding gain determined by the weighted square product distance (WSPD). A new design criterion for HSTTCs has been proposed and the optimum 4-state code, based on this criterion, has been found by systematic code search. Simulation results are provided for the quaternary phase-shift keying (QPSK) signal sets. They show that the proposed new code is superior by 9 dB and 11 dB to the conventional transmit antenna selection (TAS) and Tarokh-Seshadri-Calderbank codes (TSC), respectively, at a BER of 10e/sup -4/ over time-varying fading channels.