We report an observation of cavity modes in a grating plasmonic cavity sandwiching a QWIP in the wavelength range of 10–13 μm. The normal incident light is squeezed into the 0.9 μm-thick subwavelength cavity to form standing wave modes, causing the QWIP photocurrent responses to be 25-fold enhanced comparing with its 45-degree coupled counterpart. The cavity modes are recognized to be localized surface plasmon mode and surface plasmon polariton mode. Their dependences on grating period and strip width are investigated.