New spectroscopic observations of the double-lined eclipsing binary AQCas are presented. All available spectroscopic and photometric observations have been analysed for the fundamental properties of the components. Analyses show that the system consists of a massive primary with a mass of 17.63 +/- 0.91 Me and radius of 13.48 +/- 0.64 R-circle dot and a secondary with 12.56 +/- 0.81 M-circle dot and radius of 23.55 +/- 0.73 R-circle dot, corresponding spectral types of B0.5(+/- 2) II-III + B3(+/- 1) II. The secondary star fills its corresponding Roche lobe and mass transfer to the primary star is going on. This stream considerably does affect the photometric observations both starting from the second quarter up to the first contact of primary eclipse and just at the second maximum. Thus, the light curve is distorted and tightly depended on the wavelength of the observations. The available multi passband light curves have been analysed by taking the stream effects, as either hot or cool spots, into account. The comparison of the models and observations in the log(L/L-circle dot) log - T-eff and logg - log T-eff diagrams clearly shows that the more massive star is consistent with models and is predicted to be close to the phase of hydrogen shell ignition. Average distance to the system is estimated as 4150 +/- 240 pc using the BVJHK magnitudes and V-passband extinction. (C) 2013 Elsevier B.V. All rights reserved.