We propose a novel method of determining conduction‐band discontinuities &Dgr;Ecby using the relationship between the splitting energy &Dgr;Edue to monolayer fluctuation and quantum energy shiftEqin single‐quantum‐well (SQW) structures. This evaluation method is described by differentiating the eigenvalue equation for a finite square quantum well. The obtained formula indicates that &Dgr;Eis approximately proportional toE3/2qand that this evaluation technique is independent of ambiguity in estimation of well thickness. We apply this method to an InGaAs/InP SQW system. 4.2‐K photoluminescence spectra of InGaAs/InP SQWs grown by low‐pressure metalorganic chemical vapor deposition show clearly resolved doublets and in some cases triplets caused by monolayer fluctuations. As a result, we found that &Dgr;Ec=(0.25–0.30)&Dgr;Eggave the best fit for the InGaAs/InP heterointerface.