The experiment was conducted in a growth chamber to characterize cotton plant growth and select physiological characteristics during development of boron deficiency at the early growth stage prior to squaring. When B deficiency symptom consisting of dark bands on the petioles were first detected from boron deficient (-B) plants 4 weeks after B removal, the growth and physiology had already been affected significantly. Boron deficiency during the early growth of cotton increased leaf chlorophyll content and cell membrane leakage, decreased leaf stomatal conductance and net photosynthetic rate, and depressed plant growth and dry matter accumulation, resulting in increased fruit abscission and a change in dry matter partitioning after squaring. Therefore, in boron deficient areas, soil or foliar application of boron may improve cotton plant growth and lint yield.