Room-temperature phosphorescence (RTP) with long afterglow from pure organic materials has attracted great attention for its potential applications in biological imaging, digital encryption, optoelectronic devices, and so on. Organic materials have been long considered to be nonphosphorescent owing to their weak molecular spin-orbit coupling and high sensitivity to temperature. However, recently, some purely organic compounds have demonstrated highly efficient RTP with long afterglow upon aggregation, while others fail. Namely, it remains a challenge to expound on the underlying mechanisms. In this study, we present the molecular descriptors to characterize the phosphorescence efficiency and lifetime. For a prototypical RTP system consisting of a carbonyl group and pi-conjugated segments, the excited states can be regarded as an admixture of n -> pi* (with portion alpha) and pi -> pi* (portion beta). Starting from the phosphorescent process and El-Sayed rule, we deduced that (i) the intersystem crossing (ISC) rate of S-1 -> T-n is mostly governed by the modification of the product of alpha and beta and (ii) the ISC rate of T-1 -> S-0 is determined by the beta value of T-1. Thus, the descriptors (gamma = alpha x beta, beta) can be employed to describe the RTP character of organic molecules. From hybrid quantum mechanics and molecular mechanics (QM/MM) calculations, we illustrated the relationships among the descriptors (gamma, beta), phosphorescence efficiency and lifetime, and spin-orbit coupling constants. We stressed that the large gamma and beta values are favorable for the strong and long-lived RTP in organic materials. Experiments have reported confirmations of these molecular design rules.