Formal usability evaluation of Mixed Reality (MR) environments to date has received very little attention in MR literature and research. While awareness of the need for usability evaluation of MR appears to be on the rise, the techniques needed to perform efficient, meaningful usability evaluation of these environments are not yet available. This paper presents how the usability of MR system interfaces could be evaluated by using a combination of heuristic evaluation and formative user testing. This paper demonstrates by applying in assessing an MR-based collaborative virtual environment prototype for mechanical design review - Mixed Reality-based collaborative virtual environment (MRCVE). The evaluation was implemented through a devised experiment where human subjects were invited to use the MRCVE system for identifying design errors collaboratively from virtual mechanical models. Usability suggestions were also identified for the interface of future versions of MRCVE.