This study aimed to determine the Wheat response to nitrogen topdressing doses and calibrate the optic sensör NDVI reading. The objective of this study was to quantify and validate the relationships between N uptake and grain yield of wheat using in-season measurements by chlorophyll meter and GreenSeeker optical sensor at Zadoks3.0 growth stage (stem elongation) of wheat. In the study, responses of four winter wheat cultivars (Altay2000, Gerek79, Sönmez 2001 and Bezostaja1) to six N rate (0,3,6,9,12 and 15 kgNda -1 ) under rainfed conditions were compared with vegetation indices based on spectral reflection and In-Season Estimates of Yield calculated from these indices. GreenSeekersensor was used for this purpose. Vegetation indices (NDVI) were obtained at growth stages Zadoks2.4, Zadoks3.0, Zadoks3.1 and Zadoks3.2, Zadoks3.0 (stem elongation) was found to be the most realistic reading time. In the last year of the