For a domain A containing a field k with tr.degk A < ∞, we define a new transcendence degree of A with respect to k, which is denoted by tdk A. By using this, we generalize the theorem that for every affine domain A over a field k it holds that dim A = tr.degk A. For example, we show that if A is a quasi-local domain containing a field k with dim A = tdk A < ∞, then for every Noetherian local k-subalgebra R of A it holds that dim R = tdk R. Moreover we also generalize the theorem due to Gilmer, Nashier and Nichols.