Cybersickness is still an inevitable adverse effect when using VR systems, resulting in different levels of discomfort, and potentially breaking the immersive experience. To date, few studies have focused on exploring VR experience in the supine posture. Recent research indicated that simple adop- tion of VR game initially designed for a seated posture (by rotating 90 degrees) could lead to more severe cybersickness, even to experienced users. Following the insights from previous literature and the widely-accepted sensory conflict theory, we proposed an assumption that might explain such a phenomenon. The hypothesis is that when the perceived virtual coordinate system contradicts the received real-world coordinate through our vestibular system, the conflict appears, which can lead to a sense of discomfort. Hence, the primary goal of the study is to evaluate whether such conflict has an impact on cybersickness. Furthermore, we explored methods of mitigating this conflict through different game designs so as to improve the experience for the supine posture. The final results show that the design that aligned with the real vertical axis is effective in mitigating cybersickness, especially for games that present an acceleration sensation.