Room-temperature phosphorescence (RTP) emitters with ultralong lifetimes are attracting more and more attention for their wide applications. However, it is still a big challenge to achieve persistent organic afterglow because of the undefined relationship between molecular structures and RTP effect. Herein, diphenylamine (DPA) as a commonly used building block is selected as the molecular skeleton. Through incorporation of various alkyl moieties by ortho-substitution in different numbers and positions, RTP lifetimes can increase from 129 to 661 ms with the subtle adjustment of molecular conformations. It is summarized that the deviation angle (θ) of phenyl units in the DPA skeleton from the ideal p-π conjugated plane can act as the key parameter determining RTP lifetime, and the larger the θ values, the longer the RTP lifetimes. Furthermore, this result has been successfully applied as the universal principle to explain the RTP properties of various organic luminogens with DPA blocks and similar structures.