We present the results of a study of frequency upconversion of femtosecond optical pulses in a step-index polymer optical fiber that uses a stilbenoid compound as an active dopant. Intense blue emission is obvserved in the doped poly(methyl methacrylate) (PMMA) fiber when it is longitudinally pumped at 800 nm by 175 fs optical pulses. By means of the intensity-dependent transmission method, the two-photon absorption cross-section is deduced. Our study illustrates that the combination of a well-designed organic chromophore incorporate into a fiber geometry is appealing for the development of an upconversion blue polymer laser.