The industrial use of nanoparticles is rapidly increasing in agricultural products, and it causes numerous effects on plant growth and seed germination. There are limited studies about the uptake, accumulation, and effect of nanoparticles on plant growth. Moreover, there are no studies that have been able to measure the biological activity of seeds’ interiors under nanoparticle treatment prior to germination. Hence, in this study, the possible size-defendant effect of alumina nanoparticles on lentil (Lens culinaris) seed germination was monitored using biospeckle Optical Coherence Tomography (bOCT) at an early stage before germination. bOCT is a non-contact, non-destructive, and non-invasive image modality developed by researchers to visualize internal biological activities in vivo. Previous studies by researchers using the technique have demonstrated its potential to observe the effect of different stimuli on seed germination at an early stage before germination occurs. In the present study, the possible size-defendant effect of 200 nm and 2000 nm alumina nanoparticles (AlNPs) on lentil seed germination was examined at a concentration of 100 mg/L. The results imply that the AlNPs could enhance lentil seed germination and seedling growth compared to control treatments, and the effect is more significant as the particle size decreases. This is believed to be due to the enhancement of energy metabolism under exposure to AlNPs.