fAPAR_canopy is an important ecophysiological parameter, widely used for Gross Primary Production estimation. The paper presents the effect of diurnal changes in fAPAR_canopy on two spectral vegetation indices – NDVI and PRI. Measurements were made on 28th of May, 2012, for 4 measurement plots (fallow, spring barley, winter rye, winter wheat). Measurements of incident, transmitted and reflected photosynthetically active radiation (used for fAPAR_canopy calculation) were performed using Delta-T measurements system (Delta-T Devices, Cambridge, UK). Measurements of incident and reflected radiation at 530, 570, 670 and 849 nm were used for NDVI and PRI calculation. They were performed using two SKR1850 sensors (SKYE Instruments Ltd., Llandrindod Wells, UK) mounted in zenith-nadir position on the mobile Spectrosense 2+ device. Presented data showed that NDVI was much better than PRI spectral indicator of diurnal variability of  fAPAR_canopy of crop plants. Results indicated that the fraction of absorbed photosyntetically active radiation (although its impact was noticeable) was not the only factor affecting diurnal changes in NDVI and PRI values.