This paper describes a platform package, called "MR Platform," which we have been implementing for research and development of augmented reality technology and applications. This package includes a parallax-less stereo video see-through HMD and a software development kit (SDK) for a Linux PC environment. The SDK is composed of a C++ class library for making runtime MR applications and related utilities such as a camera calibration tool. By using the SDK, the following functions are available; capturing video, handling a six degree-of-freedom (DOF) sensor, image processing such as color detection, estimating head position and orientation, displaying the real world image, and calibrating sensor placement and camera parameters of two cameras mounted on the HMD.