IGRINS (Immersion Grating Infrared Spectrometer) is a cross-dispersed high resolution near-infrared spectrograph whose primary disperser is a silicon immersion grating (SIG) and cross-dispersers are two volume phase holographic gratings (VPHG). IGRINS covers the full ranges of H and K astronomical wavelength bands at a single exposure with the spectral resolution of 40,000. The overall layout of the IGRINS Cryostat is a $960{ imes}600{ imes}380$ cubic millimeter rectangular box and the whole optical train is sitting on an $880{ imes}520{ imes}50;mm^3$ rectangular Optical Bench. The total volume of the instrument has been revolutionarily reduced and remained compact for the spectral coverage and sensitivity of a high resolution spectrograph in infrared. We, in this presentation, introduce the design models, the structural and thermal analysis results of the mechanics and cryogenics of IGRINS.