The word length effect in speech production refers to the phenomenon that monosyllable word reaction time is obviously shorter than polysyllable’s during the picture naming or word decision process. The previous researchers summed up this kind of effect caused by the difference brought from the phonological coding stage. And they considered the phenomenon as the evidence of the view of incremental phonological encoding and planning. But the theory states that the language production includes three main processes such as the concept activation, the lexical selection and the phonological encoding. Is phonological encoding process the only cause of word length effect? What is the real relationship between the word length effect and three main processes in language production? Prime naming task was adopted in both experiment 1 and 2 (former used box prime and oracle prime used for the latter) to investigate whether the lexical selection process influenced the emerging of word length effect. The results showed that the reaction time of word naming between congruent prime and incongruent prime had no significant difference, which meant the lexical process did not impact the word length effect. In experiment 3, picture naming paradigm was used to test the relationship among concept encoding, phonological encoding and the word length effect. Thirty-two Pictures were selected, each of them could be named in 4 different patterns (basic concept level one-character word, basic concept level two-character word1, basic concept level two-characters word2, subordinate concept level two-character word). The reaction time was compared across different concept level and different word length. Result indicated that the main effect of concept level and word length are both significant, response time of subordinate concept level word was significantly longer than basic level word, response time of two-character word was significantly longer than one-character word. Therefore, both concept activation process and phonological encoding process have impact on the word length effect. In conclusion, the experiment results showed that among three different language production stages, both the concept activation and the phonological coding process are the direct causes of word length effect. The findings are consistent with incremental phonological encoding and planning as well as the basic-level category theory.