Rice flag leaf functions as the panicle's main source of photosynthates contributing to grain filling. About 90% of rice grain yield depends on the photosynthesis rate after flowering, and over 50% of the carbohydrates that accumulate in grains are produced by flag leaves. The present study focuses on capturing variability of chlorophyll concentration (CC) in flag leaves of a set of 280 diverse rice germplasm accessions during the booting and heading stages. The experiment was conducted during 2023 following an augmented block design with three checks (Presidio, Trinity, and RU2003220) at Texas A&M AgriLife Research Center in Beaumont, Texas, USA. Flag leaf CC data was collected twice during booting and 50% heading stage by an MC-100 chlorophyll meter. The diverse rice accessions set showed wide variability for CC concentration in both booting and heading growth stages. The population mean of CC at booting was 308.5 micromoles per m2 (µmol m2), ranged from 24.3 to 554.7 µmol m2. The CC ranged from 47.0 to 680.7 with a mean of 394.6 µmol m2 at 50% heading. The experiment is ongoing in the field and data collection will be continued on Flag leaf length, width, and area. The population is underway for genotyping with GBSSNPs. With all the genotype and phenotype data, GWAS analysis will be accomplished to identify significant marker trait associations for various flag leaf attributes and CC. At the end of this study, germplasms with high CC and better flag leaf will be identified and integrated into the breeding program. Markers associated with positive attributes of those traits will be identified and validated for application in marker assisted selection.