In the developed 28 GHz radio over fiber (RoF) transmission system, an eye diagram of the recovered baseband data changes between opening and closing with time after transmission through an optical fiber, which results in a destruction of the communication reliability. After a detailed experimental investigation and theoretical analysis on this phenomenon, we find the propagation delay of radio frequency (RF) signals changes with the temperature variation of the optical fiber, which can extremely destroy the communication reliability, especially for the RoF system with a high-frequency carrier and a long-distance transmission. In this paper, we propose and experimentally demonstrate an improved RoF system to overcome the phase-shift variation based on the carrier recovery technique. We employ a dual down-conversion structure and extract synchronous clock from the intermediate frequency signal to acquire a stable recovered original baseband data. A low bit error rate ( less than 10 -9) is achieved after transmission over a 25 km single mode fiber in this improved 28 GHz RoF system with a data signal of 1 Gb/s.