This study aimed to determine the Wheat response to nitrogen topdressing doses and calibrate the optic sensor 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, Sonmez 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 project NDVI based calibration equations were preferred for tests in actual farmers' fields due to the fact that NDVI method had an advantage, over Spadmeter (SPAD) use, of giving information on biomass, in addition to nitrogen nutrition status of the crop, making in-season yield estimation possible. A comparison of the system with traditional farmer applications, based on the average of 8 farmers' fields, the new system was shown to give similar yields with 0.9 kg da(-1) less Nitrogen in the spring (ZD3.0), showing its economically promising value. Chlorophyll Sufficiency Index was determined by using spadmeter in the early spring nitrogen fertilizer recommendation in the same period as 0.95 critical threshold value which showed increase in yield caused by increase in NSPAD values.