This paper presents a cross-lingual voice conversion approach using bilingual Phonetic PosteriorGram (PPG) and average modeling. The proposed approach makes use of bilingual PPGs to represent speaker-independent features of speech signals from different languages in the same feature space. In particular, a bilingual PPG is formed by stacking two monolingual PPG vectors, which are extracted from two monolingual speech recognition systems. The conversion model is trained to learn the relationship between bilingual PPGs and the corresponding acoustic features. To leverage the linguistic and acoustic information from other speakers in different languages, an average model is trained with multiple speakers in both source and target languages. I-vector is utilized as an additional input feature of the average model for network adaptation. Experiments are performed for intralingual and cross-lingual voice conversion between English and Mandarin speakers. Both objective and subjective evaluations demonstrate the effectiveness of our proposed approach.