The study aimed at revealing the extent of availability of critical thinking skills in the constructive activities of Islamic Education books for eleven and twelve grades in the Sultanate of Oman. For the achievement of this goal, the reseacher has prepared a list of critical thinking proper to students of eleven and twelve grades distributed into four main skills of critical thinking which are : investigation, deduction, conclusion and evalution. The sample study consisted in constructive activities in the books of Islamic Education for eleven and twelve grades for the accademic year 2013/2014. The number of constructive activities reached 243 activities that were analysed in the light of the following four critical thinking skills: Investigation Deduction Conclusion Evaluation The study reached the following main results: The most available critical thinking skills in the constructive activities are the conclusion skill that reached 35.3 per cent followed by the investigation skill which reached 32.51 per cent. Then the deduction skill that reached 23.86 per cent. Finally, the evaluation skill which reached 8.53 per cent The availability of the main critical thinking skills varies according to the grades. In the light of the results, the study concludes that critical thinking skills in constructive activities should comply with the critical thinking skills of the constructive activities of eleven and twelve grades and should contain the four critical thinking skills (investigation, deduction, conclusion and evaluation) in the constructive activities of the Islamic Education book