Complement constructions vary significantly in English and Persian. There are more complementation structures in English than in Persian and a complement structure in Persian might have more than one equivalent in English. Producing complement structures (CSs) in English is very difficult for native speakers of Persian, especially in an EFL context. Some studies conducted in this field including that of Hart & Schacter (1986) on the subjects with diverse linguistic backgrounds such as Persian, Spanish, Arabic, Japanese and Chinese (Schwarte 1992), Anderson's study (1993) on Spanish and Persian learners learning English as a second language, and Sepassi & Marzban (2005) support the fact that Persian native speakers have many difficulties in handling English Complement (EC) constructions. Rohde (2002) and Vercellotti & Jong (2006) believe that infinitival and gerundive verb complements can pose difficulties for L2 learners. This study aims to investigate EFL Persian learners' problems in producing English Sentential Complements (ESCs). Errors made by these learners, the hierarchy of difficulty order in processing ESCs, and probable interference of the learners' mother tongue in producing these sentences are researched. After giving a placement test to 1,200 freshmen studying in various fields of study in Shiraz University, fifty subjects of intermediate proficiency level were chosen to take a one hundred-item test including five different production tasks on 12 sub-categorization and five syntactic rules of ESCs. To work out the sequence of difficulty, the Ordering-Theoretic Method developed by Bart & Krus (1993) was utilized. The findings revealed a difficulty order in which the Infinitive-END/Gerund and Infinitive-END/That were the easiest, while For/To, Raising, Possessive and Tense-Sequencing were the most difficult types of ESCs. The results also showed that there were some prerequisite relationships among complement categories. Yet, ESCs parallel to their Persian equivalents were easier to produce than contrasting structures. These findings indicate that the factors leading to ease or difficulty of the production of ESCs include Economy Strategy, Native Language Transfer Strategy, and Environment (i.e. NP or END) of the verbs in the given context. (Contains 6 tables and 1 figure.)