The optical absorption spectra of intersubband transitions in In0.3Ga0.7As/GaAs multiple quantum dots (MQDs) grown by molecular beam epitaxy were investigated. By varying the number of In0.3Ga0.7As monolayers deposited, a series of samples with varying dot sizes ranging from 10 – 50 monolayers were obtained. The quantum dots grown with size less than 15 monolayers or more than 50 monolayers did not yield any observable measurements of intersubband transition. This suggests that there exist a critical upper and lower limit of In0.3Ga0.7As quantum dots for infrared detectors. A wavelength range of 8.60 – 13.70 μm is achieved for structures grown with the above monolayers range. The theoretical line-shape of the intersubband transition absorption was compared to the experimental measurements. From the lineshape, it was deduced that bound-to-continuum transtition is present in thick quantum dots and bound-to-bound transition is present in thinly grown quantum dots.