Flux pinning by screw dislocations and (oxygen) vacancies is considered in Y1Ba2Cu3OT_6 thin films. Expressions for the corresponding elementary pinning forces are given and estimates concerning their strengths are made. It is supposed that the screw dislocations are the predominant pinning mechanism up to a (low) field value Bsc where all strong pinning sites due to the screw dislocations are occupied. At high fields, (oxygen) vacancies in the CuO2 planes are proposed to be the main source of pinning. The different temperature dependences may be used to discriminate between these pinning mechanisms. In order to check this model, the critical current was measured on a film with a certain screw dislocation density by means of transport and irreversible magnetization measurements. At low fields (0.05 and 0.075 T), the temperature dependence of measured critical current density could be well described by pinning due to screw dislocations, whereas at higher fields (up to 6 T) the temperature dependence resembled that for pinning due to oxygen vacancies, though the agreement was less satisfactory. We conclude that the assumptions concerning the pinning behaviour were reasonably confirmed.